Provided by: libxml-xql-perl_0.68-8ubuntu1_all bug


       XML::XQL::Tutorial - Describes the XQL query syntax


       This document describes basic the features of the XML Query Language (XQL.)  A proposal
       for the XML Query Language (XQL) specification was submitted to the XSL Working Group in
       September 1998.  The spec can be found at <>.
       Since it is only a proposal at this point, things may change, but it is very likely that
       the final version will be close to the proposal.  Most of this document was copied
       straight from the spec.

       See also the XML::XQL man page.


       XQL (XML Query Language) provides a natural extension to the XSL pattern language. It
       builds upon the capabilities XSL provides for identifying classes of nodes, by adding
       Boolean logic, filters, indexing into collections of nodes, and more.

       XQL is designed specifically for XML documents.  It is a general purpose query language,
       providing a single syntax that can be used for queries, addressing, and patterns.  XQL is
       concise, simple, and powerful.

       XQL is designed to be used in many contexts. Although it is a superset of XSL patterns, it
       is also applicable to providing links to nodes, for searching repositories, and for many
       other applications.

       Note that the term XQL is a working term for the language described in this proposal. It
       is not their intent that this term be used permanently.  Also, beware that another query
       language exists called XML-QL, which uses a syntax very similar to SQL.

       The XML::XQL module has added functionality to the XQL spec, called XQL+.  To allow only
       XQL functionality as described in the spec, use the XML::XQL::Strict module. Note that the
       XQL spec makes the distinction between core XQL and XQL extensions. This implementation
       makes no distinction and the Strict module, therefore, implements everything described in
       the XQL spec.  See the XML::XQL man page for more information about the Strict module.
       This tutorial will clearly indicate when referring to XQL+.

XQL Patterns

       This section describes the core XQL notation. These features should be part of every XQL
       implementation, and serve as the base level of functionality for its use in different

       The basic syntax for XQL mimics the URI directory navigation syntax, but instead of
       specifying navigation through a physical file structure, the navigation is through
       elements in the XML tree.

       For example, the following URI means find the foo.jpg file within the bar directory:


       Similarly, in XQL, the following means find the collection of fuz elements within baz


       Throughout this document you will find numerous samples. They refer to the data shown in
       the sample file at the end of this man page.


       A context is the set of nodes against which a query operates.  For the entire query, which
       is passed to the XML::XQL::Query constructor through the Expr option, the context is the
       list of input nodes that is passed to the query() method.

       XQL allows a query to select between using the current context as the input context and
       using the 'root context' as the input context.  The 'root context' is a context containing
       only the root-most element of the document. When using XML::DOM, this is the Document

       By default, a query uses the current context. A query prefixed with '/' (forward slash)
       uses the root context. A query may optionally explicitly state that it is using the
       current context by using the './' (dot, forward slash) prefix. Both of these notations are
       analogous to the notations used to navigate directories in a file system.

       The './' prefix is only required in one situation. A query may use the '//' operator to
       indicate recursive descent. When this operator appears at the beginning of the query, the
       initial '/' causes the recursive decent to perform relative to the root of the document or
       repository. The prefix './/' allows a query to perform a recursive descent relative to the
       current context.

           Find all author elements within the current context. Since the period is really not
           used alone, this example forward-references other features:


           Note that this is equivalent to:


           Find the root element (bookstore) of this document:


           Find all author elements anywhere within the current document:


           Find all books where the value of the style attribute on the book is equal to the
           value of the specialty attribute of the bookstore element at the root of the document:

                book[/bookstore/@specialty = @style]

Query Results

       The collection returned by an XQL expression preserves document order, hierarchy, and
       identity, to the extent that these are defined.  That is, a collection of elements will
       always be returned in document order without repeats. Note that the spec states that the
       order of attributes within an element is undefined, but that this implementation does keep
       attributes in document order. See the XML::XQL man page for more details regarding
       Document Order.

Collections - 'element' and '.'

       The collection of all elements with a certain tag name is expressed using the tag name
       itself. This can be qualified by showing that the elements are selected from the current
       context './', but the current context is assumed and often need not be noted explicitly.

           Find all first-name elements. These examples are equivalent:



           Find all unqualified book elements:


           Find all elements:


Selecting children and descendants - '/' and '//'

       The collection of elements of a certain type can be determined using the path operators
       ('/' or '//'). These operators take as their arguments a collection (left side) from which
       to query elements, and a collection indicating which elements to select (right side). The
       child operator ('/')selects from immediate children of the left-side collection, while the
       descendant operator ('//') selects from arbitrary descendants of the left-side collection.
       In effect, the '//' can be thought of as a substitute for one or more levels of hierarchy.
       Note that the path operators change the context as the query is performed. By stringing
       them together users can 'drill down' into the document.

           Find all first-name elements within an author element. Note that the author children
           of the current context are found, and then first-name children are found relative to
           the context of the author elements:


           Find all title elements, one or more levels deep in the bookstore (arbitrary


           Note that this is different from the following query, which finds all title elements
           that are grandchildren of bookstore elements:


           Find emph elements anywhere inside book excerpts, anywhere inside the bookstore:


           Find all titles, one or more levels deep in the current context. Note that this
           situation is essentially the only one where the period notation is required:


Collecting element children - '*'

       An element can be referenced without using its name by substituting the '*' collection.
       The '*' collection returns all elements that are children of the current context,
       regardless of their tag name.

           Find all element children of author elements:


           Find all last-names that are grand-children of books:


           Find the grandchildren elements of the current context:


           Find all elements with specialty attributes. Note that this example uses subqueries,
           which are covered in Filters, and attributes, which are discussed in Finding an


Finding an attribute - '@'

       Attribute names are preceded by the '@' symbol. XQL is designed to treat attributes and
       sub-elements impartially, and capabilities are equivalent between the two types wherever

       Note: attributes cannot contain subelements. Thus, attributes cannot have path operators
       applied to them in a query.  Such expressions will result in a syntax error.  The XQL spec
       states that attributes are inherently unordered and indices cannot be applied to them, but
       this implementation allows it.

           Find the style attribute of the current element context:


           Find the exchange attribute on price elements within the current context:


           The following example is not valid:


           Find all books with style attributes. Note that this example uses subqueries, which
           are covered in Filters:


           Find the style attribute for all book elements:


XQL Literals

       XQL query expressions may contain literal values (i.e. constants.)  Numbers (integers and
       floats) are wrapped in XML::XQL::Number objects and strings in XML::XQL::Text objects.
       Booleans (as returned by true() and false()) are wrapped in XML::XQL::Boolean objects.

       Strings must be enclosed in single or double quotes. Since XQL does not allow escaping of
       special characters, it's impossible to create a string with both a single and a double
       quote in it. To remedy this, XQL+ has added the q// and qq// string delimiters which
       behave just like they do in Perl.

       For Numbers, exponential notation is not allowed. Use the XQL+ function eval() to
       circumvent this problem. See XML::XQL man page for details.

       The empty list or undef is represented by [] (i.e. reference to empty array) in this

           Integer Numbers:


           Floating point Numbers:



                "some text with 'single' quotes"
                'text with "double" quotes'

           Not allowed:

                1.23E-4         (use eval("1.23E-4", "Number") in XQL+)

                "can't use \"double \"quotes"  (use q/can't use "double" quotes/ in XQL+)

Grouping - '()'

       Parentheses can be used to group collection operators for clarity or where the normal
       precedence is inadequate to express an operation.

Filters - '[]'

       Constraints and branching can be applied to any collection by adding a filter clause '[ ]'
       to the collection. The filter is analogous to the SQL WHERE clause with ANY semantics. The
       filter contains a query within it, called the subquery. The subquery evaluates to a
       Boolean, and is tested for each element in the collection. Any elements in the collection
       failing the subquery test are omitted from the result collection.

       For convenience, if a collection is placed within the filter, a Boolean TRUE is generated
       if the collection contains any members, and a FALSE is generated if the collection is
       empty. In essence, an expression such as author/degree implies a collection-to-Boolean
       conversion function like the following mythical 'there-exists-a' method.


       Note that any number of filters can appear at a given level of an expression.  Empty
       filters are not allowed.

           Find all books that contain at least one excerpt element:


           Find all titles of books that contain at least one excerpt element:


           Find all authors of books where the book contains at least one excerpt, and the author
           has at least one degree:


           Find all books that have authors with at least one degree:


           Find all books that have an excerpt and a title:


   Any and all semantics - '$any$' and '$all$'
       Users can explicitly indicate whether to use any or all semantics through the $any$ and
       $all$ keywords.

       $any$ flags that a condition will hold true if any item in a set meets that condition.
       $all$ means that all elements in a set must meet the condition for the condition to hold

       $any$ and $all$ are keywords that appear before a subquery expression within a filter.

           Find all author elements where one of the last names is Bob:

                author[last-name = 'Bob']

                author[$any$ last-name = 'Bob']

           Find all author elements where none of the last-name elements are Bob:

                author[$all$ last-name != 'Bob']

           Find all author elements where the first last name is Bob:

                author[last-name[0] = 'Bob']

Indexing into a collection - '[]' and '$to$'

       XQL makes it easy to find a specific node within a set of nodes.  Simply enclose the index
       ordinal within square brackets. The ordinal is 0 based.

       A range of elements can be returned. To do so, specify an expression rather than a single
       value inside of the subscript operator (square brackets).  Such expressions can be a comma
       separated list of any of the following:

         n             Returns the nth element
         -n            Returns the element that is n-1 units from the last element.
                       E.g., -1 means the last element. -2 is the next to last element.
         m $to$ n      Returns elements m through n, inclusive

           Find the first author element:


           Find the third author element that has a first-name:


           Note that indices are relative to the parent. In other words, consider the following


           The following expression will return the first y from each of the x's:


           The following will return the first y from the entire set of y's within x's:


           The following will return the first y from the first x:


           Find the first and fourth author elements:


           Find the first through fourth author elements:

                author[0 $to$ 3]

           Find the first, the third through fifth, and the last author elements:

                author[0, 2 $to$ 4, -1]

           Find the last author element:


Boolean Expressions

       Boolean expressions can be used within subqueries. For example, one could use Boolean
       expressions to find all nodes of a particular value, or all nodes with nodes in particular
       ranges. Boolean expressions are of the form ${op}$, where {op} may be any expression of
       the form {b|a} - that is, the operator takes lvalue and rvalue arguments and returns a
       Boolean result.

       Note that the XQL Extensions section defines additional Boolean operations.

   Boolean AND and OR - '$and$' and '$or$'
       $and$ and $or$ are used to perform Boolean ands and ors.

       The Boolean operators, in conjunction with grouping parentheses, can be used to build very
       sophisticated logical expressions.

       Note that spaces are not significant and can be omitted, or included for clarity as shown

           Find all author elements that contain at least one degree and one award.

                author[degree $and$ award]

           Find all author elements that contain at least one degree or award and at least one

                author[(degree $or$ award) $and$ publication]

   Boolean NOT - '$not$'
       $not$ is a Boolean operator that negates the value of an expression within a subquery.

           Find all author elements that contain at least one degree element and that contain no
           publication elements.

                author[degree $and$ $not$ publication]

           Find all author elements that contain publications elements but do not contain either
           degree elements or award elements.

                author[$not$ (degree $or$ award) $and$ publication]

Union and intersection - '$union$', '|' and '$intersect$'

       The $union$ operator (shortcut is '|') returns the combined set of values from the query
       on the left and the query on the right. Duplicates are filtered out.  The resulting list
       is sorted in document order.

       Note: because this is a union, the set returned may include 0 or more elements of each
       element type in the list. To restrict the returned set to nodes that contain at least one
       of each of the elements in the list, use a filter, as discussed in Filters.

       The $intersect$ operator returns the set of elements in common between two sets.

           Find all first-names and last-names:

                first-name $union$ last-name

           Find all books and magazines from a bookstore:

                bookstore/(book | magazine)

           Find all books and all authors:

                book $union$ book/author

           Find the first-names, last-names, or degrees from authors within either books or

                (book $union$ magazine)/author/(first-name $union$ last-name $union$ degree)

           Find all books with author/first-name equal to 'Bob' and all magazines with price less
           than 10:

                book[author/first-name = 'Bob'] $union$ magazine[price $lt$ 10]

Equivalence - '$eq$', '=', '$ne$' and '!='

       The '=' sign is used for equality; '!=' for inequality. Alternatively, $eq$ and
        $ne$ can be used for equality and inequality.

       Single or double quotes can be used for string delimiters in expressions.  This makes it
       easier to construct and pass XQL from within scripting languages.

       For comparing values of elements, the value() method is implied. That is, last-name <
       'foo' really means last-name!value() < 'foo'.

       Note that filters are always with respect to a context. That is, the expression
       book[author] means for every book element that is found, see if it has an author
       subelement. Likewise, book[author = 'Bob'] means for every book element that is found, see
       if it has a subelement named author whose value is 'Bob'. One can examine the value of the
       context as well, by using the . (period). For example, book[. = 'Trenton'] means for every
       book that is found, see if its value is 'Trenton'.

           Find all author elements whose last name is Bob:

                author[last-name = 'Bob']

                author[last-name $eq$ 'Bob']

           Find all authors where the from attribute is not equal to 'Harvard':

                degree[@from != 'Harvard']

                degree[@from $ne$ 'Harvard']

           Find all authors where the last-name is the same as the /guest/last-name element:

                author[last-name = /guest/last-name]

           Find all authors whose text is 'Matthew Bob':

                author[. = 'Matthew Bob']

                author = 'Matthew Bob'

   Comparison - '<', '<=', '>', '>=', '$lt', '$ilt$' etc.
       A set of binary comparison operators is available for comparing numbers and strings and
       returning Boolean results.  $lt$, $le$, $gt$, $ge$ are used for less than, less than or
       equal, greater than, or greater than or equal. These same operators are also available in
       a case insensitive form: $ieq$, $ine$, $ilt$, $ile$, $igt$, $ige$.

       <, <=, > and >= are allowed short cuts for $lt$, $le$, $gt$ and $ge$.

           Find all author elements whose last name is bob and whose price is > 50

                author[last-name = 'Bob' $and$ price $gt$ 50]

           Find all authors where the from attribute is not equal to 'Harvard':

                degree[@from != 'Harvard']

           Find all authors whose last name begins with 'M' or greater:

                author[last-name $ge$ 'M']

           Find all authors whose last name begins with 'M', 'm' or greater:

                author[last-name $ige$ 'M']

           Find the first three books:

                book[index() $le$ 2]

           Find all authors who have more than 10 publications:

                author[publications!count() $gt$ 10]

   XQL+ Match operators - '$match$', '$no_match$', '=~' and '!~'
       XQL+ defines additional operators for pattern matching. The $match$ operator (shortcut is
       '=~') returns TRUE if the lvalue matches the pattern described by the rvalue. The
       $no_match$ operator (shortcut is '!~') returns FALSE if they match. Both lvalue and rvalue
       are first cast to strings.

       The rvalue string should have the syntax of a Perl rvalue, that is the delimiters should
       be included and modifiers are allowed. When using delimiters other than slashes '/', the
       'm' should be included. The rvalue should be a string, so don't forget the quotes! (Or use
       the q// or qq// delimiters in XQL+, see XML::XQL man page.)

       Note that you can't use the Perl substitution operator s/// here. Try using the XQL+
       subst() function instead.

           Find all authors whose name contains bob or Bob:

               author[first-name =~ '/[Bb]ob/']

           Find all book titles that don't contain 'Trenton' (case-insensitive):

               book[title !~ 'm!trenton!i']

   Oher XQL+ comparison operators - '$isa', '$can$'
       See the XML::XQL man page for other operators available in XQL+.

   Comparisons and vectors
       The lvalue of a comparison can be a vector or a scalar. The rvalue of a comparison must be
       a scalar or a value that can be cast at runtime to a scalar.

       If the lvalue of a comparison is a set, then any (exists) semantics are used for the
       comparison operators. That is, the result of a comparison is true if any item in the set
       meets the condition.

   Comparisons and literals
       The spec states that the lvalue of an expression cannot be a literal.  That is, '1' = a is
       not allowed. This implementation allows it, but it's not clear how useful that is.

   Casting of literals during comparison
       Elements, attributes and other XML node types are casted to strings (Text) by applying the
       value() method. The value() method calls the text() method by default, but this behavior
       can be altered by the user, so the value() method may return other XQL data types.

       When two values are compared, they are first casted to the same type.  See the XML::XQL
       man page for details on casting.

       Note that the XQL spec is not very clear on how values should be casted for comparison.
       Discussions with the authors of the XQL spec revealed that there was some disagreement and
       their implementations differed on this point.  This implementation is closest to that of
       Joe Lapp from webMethods, Inc.

Methods - 'method()' or 'query!method()'

       XQL makes a distinction between functions and methods.  See the XML::XQL man page for

       XQL provides methods for advanced manipulation of collections. These methods provide
       specialized collections of nodes (see Collection methods), as well as information about
       sets and nodes.

       Methods are of the form method(arglist)

       Consider the query book[author]. It will find all books that have authors.  Formally, we
       call the book corresponding to a particular author the reference node for that author.
       That is, every author element that is examined is an author for one of the book elements.
       (See the Annotated XQL BNF Appendix for a much more thorough definition of reference node
       and other terms. See also the XML::XQL man page.) Methods always apply to the reference

       For example, the text() method returns the text contained within a node, minus any
       structure. (That is, it is the concatenation of all text nodes contained with an element
       and its descendants.) The following expression will return all authors named 'Bob':

            author[text() = 'Bob']

       The following will return all authors containing a first-name child whose text is 'Bob':

            author[first-name!text() = 'Bob']

       The following will return all authors containing a child named Bob:

            author[*!text() = 'Bob']

       Method names are case sensitive.  See the XML::XQL man page on how to define your own
       methods and functions.

   Information methods
       The following methods provide information about nodes in a collection.  These methods
       return strings or numbers, and may be used in conjunction with comparison operators within

       Method: text()
           The text() method concatenates text of the descendents of a node, normalizing white
           space along the way. White space will be preserved for a node if the node has the
           xml:space attribute set to 'preserve', or if the nearest ancestor with the xml:space
           attribute has the attribute set to 'preserve'. When white space is normalized, it is
           normalized across the entire string. Spaces are used to separate the text between
           nodes.  When entity references are used in a document, spacing is not inserted around
           the entity refs when they are expanded.

           In this implementation, the method may receive an optional parameter to indicate
           whether the text() of Element nodes should include the text() of its Element
           descendants. See XML::XQL man page for details.


           Find the authors whose last name is 'Bob':

                author[last-name!text() = 'Bob']

           Note this is equivalent to:

                author[last-name = 'Bob']

           Find the authors with value 'Matthew Bob':

                author[text() = 'Matthew Bob']

                author[. = 'Matthew Bob']

                author = 'Matthew Bob'

       Method: rawText()
           The rawText() method is similar to the text() method, but it does not normalize

           In this implementation, the method may receive an optional parameter to indicate
           whether the rawText() of Element nodes should include the rawText() of its Element
           descendants. See XML::XQL man page for details.

       Method: value()
           Returns a type cast version of the value of a node. If no data type is provided,
           returns the same as text().

               For the purposes of comparison, value( )is implied if omitted.  In other words,
               when two items are compared, the comparison is between the value of the two items.
               Remember that in absence of type information, value() returns text().

               The following examples are equivalent:

                    author[last-name!value() = 'Bob' $and$ first-name!value() = 'Joe']

                    author[last-name = 'Bob' $and$ first-name = 'Joe']

                    price[@intl!value() = 'canada']

                    price[@intl = 'canada']

       Method: nodeType()
           Returns a number to indicate the type of the node. The values were based on the node
           type values in the DOM:

                   element         1
                   attribute       2
                   text            3
                   entity          6       (not in XQL spec)
                   PI              7
                   comment         8
                   document        9
                   doc. fragment   10      (not in XQL spec)
                   notation        11      (not in XQL spec)

           Note that in XQL, CDATASection nodes and EntityReference nodes also return 3, whereas
           in the DOM CDATASection returns 4 and EntityReference returns 5.  Use the XQL+ method
           DOM_nodeType() to get DOM node type values.  See the XML::DOM man page for node type
           values of nodes not mentioned here.

       Method: nodeTypeString
           Returns the name of the node type in lowercase or an empty string. The following node
           types are currently supported 1 (element), 2 (attribute), 3 (text), 7
           (processing_instruction), 8 (comment), 9 (document)

       Method: nodeName()
           Returns the tag name for Element nodes and the attribute name of attributes.

   Collection index methods
       Method: index()
           Returns the index of the value within the search context (i.e. with the input list of
           the subquery.) This is not necessarily the same as the index of a node within its
           parent node. Note that the XQL spec doesn't explain it well.

               Find the first 3 degrees:

                    degree[index() $lt$ 3]

               Note that it skips over other nodes that may exist between the degree elements.

               Consider the following data:


               The following expression will return the first y from each x:

                    x/y[index() = 0]

               This could also be accomplished by (see Indexing into a Collection):


       Method: end()
           The end() method returns true for the last element in the search context.  Again, the
           XQL spec does not explain it well.

               Find the last book:


               Find the last author for each book:


               Find the last author from the entire set of authors of books:


   Aggregate methods
       Method: count( [QUERY] )
           Returns the number of values inside the search context.  In XQL+, when the optional
           QUERY parameter is supplied, it returns the number of values returned by the QUERY.

   Namespace methods
       The following methods can be applied to a node to return namespace information.

       Method: baseName()
           Returns the local name portion of the node, excluding the prefix.  Local names are
           defined only for element nodes and attribute nodes.  The local name of an element node
           is the local portion of the node's element type name. The local name of an attribute
           node is the local portion of the node's attribute name. If a local name is not defined
           for the reference node, the method evaluates to the empty set.

       Method: namespace()
           Returns the URI for the namespace of the node.  Namespace URIs are defined only for
           element nodes and attribute nodes.  The namespace URI of an element node is the
           namespace URI associated with the node's element type name. The namespace URI of an
           attribute node is the namespace URI associated with the node's attribute name. If a
           namespace URI is not defined for the reference node, the method evaluates to the empty

       Method: prefix()
           Returns the prefix for the node. Namespace prefixes are defined only for element nodes
           and attribute nodes. The namespace prefix of an element node is the shortname for the
           namespace of the node's element type name.  The namespace prefix of an attribute node
           is the shortname for the namespace of the node's attribute name.  If a namespace
           prefix is not defined for the reference node, the method evaluates to the empty set.

           The spec states: A node's namespace prefix may be defined within the query expression,
           within the document under query, or within both the query expression and the document
           under query. If it is defined in both places the prefixes may not agree. In this case,
           the prefix assigned by the query expression takes precedence.  In this implementation
           you cannot define the namespace for a query, so this can never happen.

               Find all unqualified book elements. Note that this does not return my:book


               Find all book elements with the prefix 'my'. Note that this query does not return
               unqualified book elements:


               Find all book elements with a 'my' prefix that have an author subelement:


               Find all book elements with a 'my' prefix that have an author subelement with a my


               Find all elements with a prefix of 'my':


               Find all book elements from any namespace:


               Find any element from any namespace:


               Find the style attribute with a 'my' prefix within a book element:


           All attributes of an element can be returned using @*.  This is potentially useful for
           applications that treat attributes as fields in a record.

               Find all attributes of the current element context:


               Find style attributes from any namespace:


               Find all attributes from the 'my' namespace, including unqualified attributes on
               elements from the 'my' namespace:



       This section defines the functions of XQL. The spec states that: XQL defines two kinds of
       functions: collection functions and pure functions. Collection functions use the search
       context of the Invocation instance, while pure functions ignore the search context, except
       to evaluate the function's parameters. A collection function evaluates to a subset of the
       search context, and a pure function evaluates to either a constant value or to a value
       that depends only on the function's parameters.

       Don't worry if you don't get it. Just use them!

   Collection functions
       The collection functions provide access to the various types of nodes in a document. Any
       of these collections can be constrained and indexed.  The collections return the set of
       children of the reference node meeting the particular restriction.

       Function: textNode()
           The collection of text nodes.

       Function: comment()
           The collection of comment nodes.

       Function: pi()
           The collection of processing instruction nodes.

       Function: element( [NAME] )
           The collection of all element nodes. If the optional text parameter is provided, it
           only returns element children matching that particular name.

       Function: attribute( [NAME] )
           The collection of all attribute nodes. If the optional text parameter is provided, it
           only returns attributes matching that particular name.

       Function: node()
           The collection of all non-attribute nodes.

               Find the second text node in each p element in the current context:


               Find the second comment anywhere in the document. See Context for details on
               setting the context to the document root:


   Other XQL Functions
       Function: ancestor(QUERY)
           Finds the nearest ancestor matching the provided query. It returns either a single
           element result or an empty set [].  Note that this node is never the reference node

               Find the nearest book ancestor of the current element:


               Find the nearest ancestor author element that is contained in a book element:


       Function: id(NAME)
           Pure function that evaluates to a set. The set contains an element node that has an
           'id' attribute whose value is identical to the string that the Text parameter quotes.
           The element node may appear anywhere within the document under query. If more than one
           element node meets these criteria, the function evaluates to a set that contains the
           first node appearing in a document ordering of the nodes.

       Function: true() and false()
           Pure functions that each evaluate to a Boolean. "true()" evaluates to 'true', and
           "false()" evaluates to 'false'. These functions are useful in expressions that are
           constructed using entity references or variable substitution, since they may replace
           an expression found in an instance of Subquery without violating the syntax required
           by the instance of Subquery.  They return an object of type XML::XQL::Boolean.

       Function: date(QUERY)
           "date" is a pure function that typecasts the value of its parameter to a set of dates.
           If the parameter matches a single string, the value of the function is a set
           containing a single date. If the parameter matches a QUERY, the value of the function
           is a set of dates, where the set contains one date for each member of the set to which
           the parameter evaluates.

           XQL does not define the representation of the date value, nor does it define how the
           function translates parameter values into dates.  This implementation uses the
           Date::Manip module to parse dates, which accepts almost any imaginable format. See
           XML::XQL to plug in your own Date implementation.

           Include the XML::XQL::Date package to add the XQL date type and the date() function,
           like this:

            use XML::XQL::Date;

       Perl builtin functions and other XQL+ functions
           XQL+ provides XQL function wrappers for most Perl builtin functions.  It also provides
           other cool functions like subst(), map(), and eval() that allow you to modify
           documents and embed perl code.  If this is still not enough, you can add your own
           function and methods.  See XML::XQL man page for details.

Sequence Operators - ';' and ';;'

       The whitepaper 'The Design of XQL' by Jonathan Robie, which can be found at
       <> describes the sequence operators ';;'
       (precedes) and ';' (immediately precedes.) Although these operators are not included in
       the XQL spec, I thought I'd add them anyway.

   Immediately Precedes - ';'
           With the following input:

               <TD>Shady Grove</TD>
               <TD>Over the River, Charlie</TD>

           Find the TD node that contains "Shady Grove" and the TD node that immediately follows

                   //(TD="Shady Grove" ; TD)

       Note that in XML::DOM there is actually a text node with whitespace between the two TD
       nodes, but those are ignored by this operator, unless the text node has 'xml:space' set to
       'preserve'. See ??? for details.

   Precedes - ';;'
           With the following input (from Hamlet):

             <LINE>Tis gone!</LINE>
             <STAGEDIR>Exit Ghost</STAGEDIR>
             <LINE>We do it wrong, being so majestical,</LINE>
             <LINE>To offer it the show of violence;</LINE>
             <LINE>For it is, as the air, invulnerable,</LINE>
             <LINE>And our vain blows malicious mockery.</LINE>

           Return the STAGEDIR and all the LINEs that follow it:

                   SPEECH//( STAGEDIR ;; LINE )

           Suppose an actor playing the ghost wants to know when to exit; that is, he wants to
           know who says what line just before he is supposed to exit. The line immediately
           precedes the stagedir, but the speaker may occur at any time before the line.  In this
           query, we will use the "precedes" operator (";;") to identify a speaker that precedes
           the line somewhere within a speech. Our ghost can find the required information with
           the following query, which selects the speaker, the line, and the stagedir:

                   SPEECH//( SPEAKER ;; LINE ; STAGEDIR="Exit Ghost")

Operator Precedence

       The following table lists operators in precedence order, highest precedence first, where
       operators of a given row have the same precedence.  The table also lists the associated

               Production      Operator(s)
               ----------      -----------
               Grouping        ( )
               Filter          [ ]
               Subscript       [ ]
               Bang            !
               Path            / //
               Match           $match$ $no_match$ =~ !~ (XQL+ only)
               Comparison      = != < <= > >= $eq$ $ne$ $lt$ $le$ $gt$
                               $ge$ $ieq$ $ine$ $ilt$ $ile$ $igt$ $ige$
               Intersection    $intersect$
               Union           $union$ |
               Negation        $not$
               Conjunction     $and$
               Disjunction     $or$
               Sequence        ; ;;

Sample XML Document - bookstore.xml

       This file is also stored in samples/bookstore.xml that comes with the XML::XQL

        <?xml version='1.0'?>
        <!-- This file represents a fragment of a book store inventory database -->
        <bookstore specialty='novel'>
          <book style='autobiography'>
            <title>Seven Years in Trenton</title>
              <award>Trenton Literary Review Honorable Mention</award>
          <book style='textbook'>
            <title>History of Trenton</title>
                Selected Short Stories of
                <first-name>Mary</first-name> <last-name>Bob</last-name>
          <magazine style='glossy' frequency='monthly'>
            <title>Tracking Trenton</title>
            <subscription price='24' per='year'/>
          <book style='novel' id='myfave'>
            <title>Trenton Today, Trenton Tomorrow</title>
              <degree from='Trenton U'>B.A.</degree>
              <degree from='Harvard'>Ph.D.</degree>
              <publication>Still in Trenton</publication>
              <publication>Trenton Forever</publication>
            <price intl='canada' exchange='0.7'>6.50</price>
              <p>It was a dark and stormy night.</p>
              <p>But then all nights in Trenton seem dark and
              stormy to someone who has gone through what
              <emph>I</emph> have.</p>
          <my:book style='leather' price='29.50' xmlns:my=''>
            <my:title>Who's Who in Trenton</my:title>
            <my:author>Robert Bob</my:author>


       The Japanese version of this document can be found on-line at

       XML::XQL, XML::XQL::Date, XML::XQL::Query and XML::XQL::DOM