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NAME

       lists - List processing functions.

DESCRIPTION

       This module contains functions for list processing.

       Unless  otherwise  stated,  all functions assume that position numbering starts at 1. That
       is, the first element of a list is at position 1.

       Two terms T1 and T2 compare equal if T1 == T2 evaluates to true. They match if T1  =:=  T2
       evaluates to true.

       Whenever  an ordering function F is expected as argument, it is assumed that the following
       properties hold of F for all x, y, and z:

         * If x F y and y F x, then x = y (F is antisymmetric).

         * If x F y and y F z, then x F z (F is transitive).

         * x F y or y F x (F is total).

       An example of a typical ordering function is less than or equal to: =</2.

EXPORTS

       all(Pred, List) -> boolean()

              Types:

                 Pred = fun((Elem :: T) -> boolean())
                 List = [T]
                 T = term()

              Returns true if Pred(Elem) returns true for all elements Elem  in  List,  otherwise
              false.

       any(Pred, List) -> boolean()

              Types:

                 Pred = fun((Elem :: T) -> boolean())
                 List = [T]
                 T = term()

              Returns true if Pred(Elem) returns true for at least one element Elem in List.

       append(ListOfLists) -> List1

              Types:

                 ListOfLists = [List]
                 List = List1 = [T]
                 T = term()

              Returns a list in which all the sublists of ListOfLists have been appended.

              Example:

              > lists:append([[1, 2, 3], [a, b], [4, 5, 6]]).
              [1,2,3,a,b,4,5,6]

       append(List1, List2) -> List3

              Types:

                 List1 = List2 = List3 = [T]
                 T = term()

              Returns  a new list List3, which is made from the elements of List1 followed by the
              elements of List2.

              Example:

              > lists:append("abc", "def").
              "abcdef"

              lists:append(A, B) is equivalent to A ++ B.

       concat(Things) -> string()

              Types:

                 Things = [Thing]
                 Thing = atom() | integer() | float() | string()

              Concatenates the text representation of the elements of  Things.  The  elements  of
              Things can be atoms, integers, floats, or strings.

              Example:

              > lists:concat([doc, '/', file, '.', 3]).
              "doc/file.3"

       delete(Elem, List1) -> List2

              Types:

                 Elem = T
                 List1 = List2 = [T]
                 T = term()

              Returns  a copy of List1 where the first element matching Elem is deleted, if there
              is such an element.

       droplast(List) -> InitList

              Types:

                 List = [T, ...]
                 InitList = [T]
                 T = term()

              Drops the last element of a List. The  list  is  to  be  non-empty,  otherwise  the
              function crashes with a function_clause.

       dropwhile(Pred, List1) -> List2

              Types:

                 Pred = fun((Elem :: T) -> boolean())
                 List1 = List2 = [T]
                 T = term()

              Drops  elements  Elem  from  List1  while  Pred(Elem)  returns true and returns the
              remaining list.

       duplicate(N, Elem) -> List

              Types:

                 N = integer() >= 0
                 Elem = T
                 List = [T]
                 T = term()

              Returns a list containing N copies of term Elem.

              Example:

              > lists:duplicate(5, xx).
              [xx,xx,xx,xx,xx]

       filter(Pred, List1) -> List2

              Types:

                 Pred = fun((Elem :: T) -> boolean())
                 List1 = List2 = [T]
                 T = term()

              List2 is a list of all elements Elem in List1 for which Pred(Elem) returns true.

       filtermap(Fun, List1) -> List2

              Types:

                 Fun = fun((Elem) -> boolean() | {true, Value})
                 List1 = [Elem]
                 List2 = [Elem | Value]
                 Elem = Value = term()

              Calls Fun(Elem) on successive elements Elem of List1. Fun/1 must  return  either  a
              Boolean  or  a  tuple  {true, Value}. The function returns the list of elements for
              which Fun returns a new value, where a value of  true  is  synonymous  with  {true,
              Elem}.

              That is, filtermap behaves as if it had been defined as follows:

              filtermap(Fun, List1) ->
                  lists:foldr(fun(Elem, Acc) ->
                                     case Fun(Elem) of
                                         false -> Acc;
                                         true -> [Elem|Acc];
                                         {true,Value} -> [Value|Acc]
                                     end
                              end, [], List1).

              Example:

              > lists:filtermap(fun(X) -> case X rem 2 of 0 -> {true, X div 2}; _ -> false end end, [1,2,3,4,5]).
              [1,2]

       flatlength(DeepList) -> integer() >= 0

              Types:

                 DeepList = [term() | DeepList]

              Equivalent to length(flatten(DeepList)), but more efficient.

       flatmap(Fun, List1) -> List2

              Types:

                 Fun = fun((A) -> [B])
                 List1 = [A]
                 List2 = [B]
                 A = B = term()

              Takes  a  function  from As to lists of Bs, and a list of As (List1) and produces a
              list of Bs by applying the function to every element in  List1  and  appending  the
              resulting lists.

              That is, flatmap behaves as if it had been defined as follows:

              flatmap(Fun, List1) ->
                  append(map(Fun, List1)).

              Example:

              > lists:flatmap(fun(X)->[X,X] end, [a,b,c]).
              [a,a,b,b,c,c]

       flatten(DeepList) -> List

              Types:

                 DeepList = [term() | DeepList]
                 List = [term()]

              Returns a flattened version of DeepList.

       flatten(DeepList, Tail) -> List

              Types:

                 DeepList = [term() | DeepList]
                 Tail = List = [term()]

              Returns a flattened version of DeepList with tail Tail appended.

       foldl(Fun, Acc0, List) -> Acc1

              Types:

                 Fun = fun((Elem :: T, AccIn) -> AccOut)
                 Acc0 = Acc1 = AccIn = AccOut = term()
                 List = [T]
                 T = term()

              Calls  Fun(Elem,  AccIn)  on  successive elements A of List, starting with AccIn ==
              Acc0. Fun/2 must return a new accumulator, which is passed to the  next  call.  The
              function  returns  the final value of the accumulator. Acc0 is returned if the list
              is empty.

              Example:

              > lists:foldl(fun(X, Sum) -> X + Sum end, 0, [1,2,3,4,5]).
              15
              > lists:foldl(fun(X, Prod) -> X * Prod end, 1, [1,2,3,4,5]).
              120

       foldr(Fun, Acc0, List) -> Acc1

              Types:

                 Fun = fun((Elem :: T, AccIn) -> AccOut)
                 Acc0 = Acc1 = AccIn = AccOut = term()
                 List = [T]
                 T = term()

              Like foldl/3, but the list is traversed from right to left.

              Example:

              > P = fun(A, AccIn) -> io:format("~p ", [A]), AccIn end.
              #Fun<erl_eval.12.2225172>
              > lists:foldl(P, void, [1,2,3]).
              1 2 3 void
              > lists:foldr(P, void, [1,2,3]).
              3 2 1 void

              foldl/3 is tail recursive and is usually preferred to foldr/3.

       join(Sep, List1) -> List2

              Types:

                 Sep = T
                 List1 = List2 = [T]
                 T = term()

              Inserts Sep between each element in List1. Has no effect on the empty list and on a
              singleton list. For example:

              > lists:join(x, [a,b,c]).
              [a,x,b,x,c]
              > lists:join(x, [a]).
              [a]
              > lists:join(x, []).
              []

       foreach(Fun, List) -> ok

              Types:

                 Fun = fun((Elem :: T) -> term())
                 List = [T]
                 T = term()

              Calls  Fun(Elem)  for each element Elem in List. This function is used for its side
              effects and the evaluation order is defined to be the same  as  the  order  of  the
              elements in the list.

       keydelete(Key, N, TupleList1) -> TupleList2

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = TupleList2 = [Tuple]
                 Tuple = tuple()

              Returns  a  copy  of  TupleList1  where  the  first occurrence of a tuple whose Nth
              element compares equal to Key is deleted, if there is such a tuple.

       keyfind(Key, N, TupleList) -> Tuple | false

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList = [Tuple]
                 Tuple = tuple()

              Searches the list of tuples TupleList for a tuple whose Nth element compares  equal
              to Key. Returns Tuple if such a tuple is found, otherwise false.

       keymap(Fun, N, TupleList1) -> TupleList2

              Types:

                 Fun = fun((Term1 :: term()) -> Term2 :: term())
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = TupleList2 = [Tuple]
                 Tuple = tuple()

              Returns a list of tuples where, for each tuple in TupleList1, the Nth element Term1
              of the tuple has been replaced with the result of calling Fun(Term1).

              Examples:

              > Fun = fun(Atom) -> atom_to_list(Atom) end.
              #Fun<erl_eval.6.10732646>
              2> lists:keymap(Fun, 2, [{name,jane,22},{name,lizzie,20},{name,lydia,15}]).
              [{name,"jane",22},{name,"lizzie",20},{name,"lydia",15}]

       keymember(Key, N, TupleList) -> boolean()

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList = [Tuple]
                 Tuple = tuple()

              Returns true if there is a tuple in TupleList whose Nth element compares  equal  to
              Key, otherwise false.

       keymerge(N, TupleList1, TupleList2) -> TupleList3

              Types:

                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = [T1]
                 TupleList2 = [T2]
                 TupleList3 = [T1 | T2]
                 T1 = T2 = Tuple
                 Tuple = tuple()

              Returns  the  sorted list formed by merging TupleList1 and TupleList2. The merge is
              performed on the Nth element of each tuple. Both TupleList1 and TupleList2 must  be
              key-sorted  before  evaluating  this  function.  When two tuples compare equal, the
              tuple from TupleList1 is picked before the tuple from TupleList2.

       keyreplace(Key, N, TupleList1, NewTuple) -> TupleList2

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = TupleList2 = [Tuple]
                 NewTuple = Tuple
                 Tuple = tuple()

              Returns a copy of TupleList1 where the first occurrence of  a  T  tuple  whose  Nth
              element  compares  equal to Key is replaced with NewTuple, if there is such a tuple
              T.

       keysearch(Key, N, TupleList) -> {value, Tuple} | false

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList = [Tuple]
                 Tuple = tuple()

              Searches the list of tuples TupleList for a tuple whose Nth element compares  equal
              to Key. Returns {value, Tuple} if such a tuple is found, otherwise false.

          Note:
              This function is retained for backward compatibility. Function keyfind/3 is usually
              more convenient.

       keysort(N, TupleList1) -> TupleList2

              Types:

                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = TupleList2 = [Tuple]
                 Tuple = tuple()

              Returns a list containing the  sorted  elements  of  list  TupleList1.  Sorting  is
              performed on the Nth element of the tuples. The sort is stable.

       keystore(Key, N, TupleList1, NewTuple) -> TupleList2

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = [Tuple]
                 TupleList2 = [Tuple, ...]
                 NewTuple = Tuple
                 Tuple = tuple()

              Returns  a  copy  of  TupleList1  where the first occurrence of a tuple T whose Nth
              element compares equal to Key is replaced with NewTuple, if there is such  a  tuple
              T.  If  there  is  no  such tuple T, a copy of TupleList1 where [NewTuple] has been
              appended to the end is returned.

       keytake(Key, N, TupleList1) -> {value, Tuple, TupleList2} | false

              Types:

                 Key = term()
                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = TupleList2 = [tuple()]
                 Tuple = tuple()

              Searches the list of tuples TupleList1 for a tuple whose Nth element compares equal
              to  Key.  Returns  {value,  Tuple,  TupleList2} if such a tuple is found, otherwise
              false. TupleList2 is a copy of TupleList1 where the first occurrence of  Tuple  has
              been removed.

       last(List) -> Last

              Types:

                 List = [T, ...]
                 Last = T
                 T = term()

              Returns the last element in List.

       map(Fun, List1) -> List2

              Types:

                 Fun = fun((A) -> B)
                 List1 = [A]
                 List2 = [B]
                 A = B = term()

              Takes  a  function  from  As  to  Bs, and a list of As and produces a list of Bs by
              applying the function to every element in the list. This function is used to obtain
              the return values. The evaluation order depends on the implementation.

       mapfoldl(Fun, Acc0, List1) -> {List2, Acc1}

              Types:

                 Fun = fun((A, AccIn) -> {B, AccOut})
                 Acc0 = Acc1 = AccIn = AccOut = term()
                 List1 = [A]
                 List2 = [B]
                 A = B = term()

              Combines the operations of map/2 and foldl/3 into one pass.

              Example:

              Summing the elements in a list and double them at the same time:

              > lists:mapfoldl(fun(X, Sum) -> {2*X, X+Sum} end,
              0, [1,2,3,4,5]).
              {[2,4,6,8,10],15}

       mapfoldr(Fun, Acc0, List1) -> {List2, Acc1}

              Types:

                 Fun = fun((A, AccIn) -> {B, AccOut})
                 Acc0 = Acc1 = AccIn = AccOut = term()
                 List1 = [A]
                 List2 = [B]
                 A = B = term()

              Combines the operations of map/2 and foldr/3 into one pass.

       max(List) -> Max

              Types:

                 List = [T, ...]
                 Max = T
                 T = term()

              Returns  the first element of List that compares greater than or equal to all other
              elements of List.

       member(Elem, List) -> boolean()

              Types:

                 Elem = T
                 List = [T]
                 T = term()

              Returns true if Elem matches some element of List, otherwise false.

       merge(ListOfLists) -> List1

              Types:

                 ListOfLists = [List]
                 List = List1 = [T]
                 T = term()

              Returns the sorted list formed by merging all  the  sublists  of  ListOfLists.  All
              sublists  must be sorted before evaluating this function. When two elements compare
              equal, the element from the sublist with the  lowest  position  in  ListOfLists  is
              picked before the other element.

       merge(List1, List2) -> List3

              Types:

                 List1 = [X]
                 List2 = [Y]
                 List3 = [X | Y]
                 X = Y = term()

              Returns  the  sorted  list  formed by merging List1 and List2. Both List1 and List2
              must be sorted before evaluating this function. When two  elements  compare  equal,
              the element from List1 is picked before the element from List2.

       merge(Fun, List1, List2) -> List3

              Types:

                 Fun = fun((A, B) -> boolean())
                 List1 = [A]
                 List2 = [B]
                 List3 = [A | B]
                 A = B = term()

              Returns  the  sorted  list  formed by merging List1 and List2. Both List1 and List2
              must be sorted according to  the  ordering  function  Fun  before  evaluating  this
              function.  Fun(A, B) is to return true if A compares less than or equal to B in the
              ordering, otherwise false. When two elements compare equal, the element from  List1
              is picked before the element from List2.

       merge3(List1, List2, List3) -> List4

              Types:

                 List1 = [X]
                 List2 = [Y]
                 List3 = [Z]
                 List4 = [X | Y | Z]
                 X = Y = Z = term()

              Returns  the  sorted  list formed by merging List1, List2, and List3. All of List1,
              List2, and List3 must be sorted before evaluating this function. When two  elements
              compare  equal,  the  element  from  List1,  if there is such an element, is picked
              before the other element, otherwise the element from List2  is  picked  before  the
              element from List3.

       min(List) -> Min

              Types:

                 List = [T, ...]
                 Min = T
                 T = term()

              Returns  the  first  element  of List that compares less than or equal to all other
              elements of List.

       nth(N, List) -> Elem

              Types:

                 N = integer() >= 1
                   1..length(List)
                 List = [T, ...]
                 Elem = T
                 T = term()

              Returns the Nth element of List.

              Example:

              > lists:nth(3, [a, b, c, d, e]).
              c

       nthtail(N, List) -> Tail

              Types:

                 N = integer() >= 0
                   0..length(List)
                 List = [T, ...]
                 Tail = [T]
                 T = term()

              Returns the Nth tail of List, that is, the sublist of  List  starting  at  N+1  and
              continuing up to the end of the list.

              Example

              > lists:nthtail(3, [a, b, c, d, e]).
              [d,e]
              > tl(tl(tl([a, b, c, d, e]))).
              [d,e]
              > lists:nthtail(0, [a, b, c, d, e]).
              [a,b,c,d,e]
              > lists:nthtail(5, [a, b, c, d, e]).
              []

       partition(Pred, List) -> {Satisfying, NotSatisfying}

              Types:

                 Pred = fun((Elem :: T) -> boolean())
                 List = Satisfying = NotSatisfying = [T]
                 T = term()

              Partitions  List  into  two  lists,  where the first list contains all elements for
              which Pred(Elem) returns true, and the second list contains all elements for  which
              Pred(Elem) returns false.

              Examples:

              > lists:partition(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).
              {[1,3,5,7],[2,4,6]}
              > lists:partition(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).
              {[a,b,c,d,e],[1,2,3,4]}

              For a different way to partition a list, see splitwith/2.

       prefix(List1, List2) -> boolean()

              Types:

                 List1 = List2 = [T]
                 T = term()

              Returns true if List1 is a prefix of List2, otherwise false.

       reverse(List1) -> List2

              Types:

                 List1 = List2 = [T]
                 T = term()

              Returns a list with the elements in List1 in reverse order.

       reverse(List1, Tail) -> List2

              Types:

                 List1 = [T]
                 Tail = term()
                 List2 = [T]
                 T = term()

              Returns  a  list  with  the  elements  in  List1  in  reverse order, with tail Tail
              appended.

              Example:

              > lists:reverse([1, 2, 3, 4], [a, b, c]).
              [4,3,2,1,a,b,c]

       search(Pred, List) -> {value, Value} | false

              Types:

                 Pred = fun((T) -> boolean())
                 List = [T]
                 Value = T

              If there is a Value in List such that Pred(Value)  returns  true,  returns  {value,
              Value} for the first such Value, otherwise returns false.

       seq(From, To) -> Seq

       seq(From, To, Incr) -> Seq

              Types:

                 From = To = Incr = integer()
                 Seq = [integer()]

              Returns  a  sequence  of integers that starts with From and contains the successive
              results of adding Incr to the previous element, until To is reached or  passed  (in
              the latter case, To is not an element of the sequence). Incr defaults to 1.

              Failures:

                * If To < From - Incr and Incr > 0.

                * If To > From - Incr and Incr < 0.

                * If Incr =:= 0 and From =/= To.

              The following equalities hold for all sequences:

              length(lists:seq(From, To)) =:= To - From + 1
              length(lists:seq(From, To, Incr)) =:= (To - From + Incr) div Incr

              Examples:

              > lists:seq(1, 10).
              [1,2,3,4,5,6,7,8,9,10]
              > lists:seq(1, 20, 3).
              [1,4,7,10,13,16,19]
              > lists:seq(1, 0, 1).
              []
              > lists:seq(10, 6, 4).
              []
              > lists:seq(1, 1, 0).
              [1]

       sort(List1) -> List2

              Types:

                 List1 = List2 = [T]
                 T = term()

              Returns a list containing the sorted elements of List1.

       sort(Fun, List1) -> List2

              Types:

                 Fun = fun((A :: T, B :: T) -> boolean())
                 List1 = List2 = [T]
                 T = term()

              Returns  a  list containing the sorted elements of List1, according to the ordering
              function Fun. Fun(A, B) is to return true if A compares less than or equal to B  in
              the ordering, otherwise false.

       split(N, List1) -> {List2, List3}

              Types:

                 N = integer() >= 0
                   0..length(List1)
                 List1 = List2 = List3 = [T]
                 T = term()

              Splits  List1  into  List2 and List3. List2 contains the first N elements and List3
              the remaining elements (the Nth tail).

       splitwith(Pred, List) -> {List1, List2}

              Types:

                 Pred = fun((T) -> boolean())
                 List = List1 = List2 = [T]
                 T = term()

              Partitions List into two lists according to Pred. splitwith/2 behaves as if  it  is
              defined as follows:

              splitwith(Pred, List) ->
                  {takewhile(Pred, List), dropwhile(Pred, List)}.

              Examples:

              > lists:splitwith(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).
              {[1],[2,3,4,5,6,7]}
              > lists:splitwith(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).
              {[a,b],[1,c,d,2,3,4,e]}

              For a different way to partition a list, see partition/2.

       sublist(List1, Len) -> List2

              Types:

                 List1 = List2 = [T]
                 Len = integer() >= 0
                 T = term()

              Returns  the  sublist  of  List1  starting  at  position  1  and with (maximum) Len
              elements. It is not an error for Len to exceed the length of the list, in that case
              the whole list is returned.

       sublist(List1, Start, Len) -> List2

              Types:

                 List1 = List2 = [T]
                 Start = integer() >= 1
                   1..(length(List1)+1)
                 Len = integer() >= 0
                 T = term()

              Returns  the sublist of List1 starting at Start and with (maximum) Len elements. It
              is not an error for Start+Len to exceed the length of the list.

              Examples:

              > lists:sublist([1,2,3,4], 2, 2).
              [2,3]
              > lists:sublist([1,2,3,4], 2, 5).
              [2,3,4]
              > lists:sublist([1,2,3,4], 5, 2).
              []

       subtract(List1, List2) -> List3

              Types:

                 List1 = List2 = List3 = [T]
                 T = term()

              Returns a new list List3 that is a  copy  of  List1,  subjected  to  the  following
              procedure: for each element in List2, its first occurrence in List1 is deleted.

              Example:

              > lists:subtract("123212", "212").
              "312".

              lists:subtract(A, B) is equivalent to A -- B.

       suffix(List1, List2) -> boolean()

              Types:

                 List1 = List2 = [T]
                 T = term()

              Returns true if List1 is a suffix of List2, otherwise false.

       sum(List) -> number()

              Types:

                 List = [number()]

              Returns the sum of the elements in List.

       takewhile(Pred, List1) -> List2

              Types:

                 Pred = fun((Elem :: T) -> boolean())
                 List1 = List2 = [T]
                 T = term()

              Takes elements Elem from List1 while Pred(Elem) returns true, that is, the function
              returns the longest  prefix  of  the  list  for  which  all  elements  satisfy  the
              predicate.

       ukeymerge(N, TupleList1, TupleList2) -> TupleList3

              Types:

                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = [T1]
                 TupleList2 = [T2]
                 TupleList3 = [T1 | T2]
                 T1 = T2 = Tuple
                 Tuple = tuple()

              Returns  the  sorted list formed by merging TupleList1 and TupleList2. The merge is
              performed on the Nth element of each tuple. Both TupleList1 and TupleList2 must  be
              key-sorted  without  duplicates  before  evaluating  this function. When two tuples
              compare equal, the tuple from TupleList1 is picked and the one from  TupleList2  is
              deleted.

       ukeysort(N, TupleList1) -> TupleList2

              Types:

                 N = integer() >= 1
                   1..tuple_size(Tuple)
                 TupleList1 = TupleList2 = [Tuple]
                 Tuple = tuple()

              Returns  a  list containing the sorted elements of list TupleList1 where all except
              the first tuple of the  tuples  comparing  equal  have  been  deleted.  Sorting  is
              performed on the Nth element of the tuples.

       umerge(ListOfLists) -> List1

              Types:

                 ListOfLists = [List]
                 List = List1 = [T]
                 T = term()

              Returns  the  sorted  list  formed  by merging all the sublists of ListOfLists. All
              sublists must be sorted and contain no duplicates before evaluating this  function.
              When  two  elements  compare  equal,  the  element from the sublist with the lowest
              position in ListOfLists is picked and the other is deleted.

       umerge(List1, List2) -> List3

              Types:

                 List1 = [X]
                 List2 = [Y]
                 List3 = [X | Y]
                 X = Y = term()

              Returns the sorted list formed by merging List1 and List2.  Both  List1  and  List2
              must  be sorted and contain no duplicates before evaluating this function. When two
              elements compare equal, the element from List1 is picked and the one from List2  is
              deleted.

       umerge(Fun, List1, List2) -> List3

              Types:

                 Fun = fun((A, B) -> boolean())
                 List1 = [A]
                 List2 = [B]
                 List3 = [A | B]
                 A = B = term()

              Returns  the  sorted  list  formed by merging List1 and List2. Both List1 and List2
              must be sorted according to the ordering function Fun  and  contain  no  duplicates
              before  evaluating  this  function.  Fun(A, B) is to return true if A compares less
              than or equal to B in the ordering, otherwise  false.  When  two  elements  compare
              equal, the element from List1 is picked and the one from List2 is deleted.

       umerge3(List1, List2, List3) -> List4

              Types:

                 List1 = [X]
                 List2 = [Y]
                 List3 = [Z]
                 List4 = [X | Y | Z]
                 X = Y = Z = term()

              Returns  the  sorted  list formed by merging List1, List2, and List3. All of List1,
              List2, and List3 must be sorted and contain no duplicates  before  evaluating  this
              function.  When  two  elements  compare  equal, the element from List1 is picked if
              there is such an element, otherwise the element from List2 is picked, and the other
              is deleted.

       unzip(List1) -> {List2, List3}

              Types:

                 List1 = [{A, B}]
                 List2 = [A]
                 List3 = [B]
                 A = B = term()

              "Unzips"  a  list  of  two-tuples into two lists, where the first list contains the
              first element of each tuple, and the second list contains  the  second  element  of
              each tuple.

       unzip3(List1) -> {List2, List3, List4}

              Types:

                 List1 = [{A, B, C}]
                 List2 = [A]
                 List3 = [B]
                 List4 = [C]
                 A = B = C = term()

              "Unzips" a list of three-tuples into three lists, where the first list contains the
              first element of each tuple, the second list contains the second  element  of  each
              tuple, and the third list contains the third element of each tuple.

       usort(List1) -> List2

              Types:

                 List1 = List2 = [T]
                 T = term()

              Returns  a  list containing the sorted elements of List1 where all except the first
              element of the elements comparing equal have been deleted.

       usort(Fun, List1) -> List2

              Types:

                 Fun = fun((T, T) -> boolean())
                 List1 = List2 = [T]
                 T = term()

              Returns a list containing the sorted elements of List1 where all except  the  first
              element of the elements comparing equal according to the ordering function Fun have
              been deleted. Fun(A, B) is to return true if A compares less than or equal to B  in
              the ordering, otherwise false.

       zip(List1, List2) -> List3

              Types:

                 List1 = [A]
                 List2 = [B]
                 List3 = [{A, B}]
                 A = B = term()

              "Zips"  two  lists  of  equal  length  into one list of two-tuples, where the first
              element of each tuple is taken from the first list and the second element is  taken
              from the corresponding element in the second list.

       zip3(List1, List2, List3) -> List4

              Types:

                 List1 = [A]
                 List2 = [B]
                 List3 = [C]
                 List4 = [{A, B, C}]
                 A = B = C = term()

              "Zips"  three  lists of equal length into one list of three-tuples, where the first
              element of each tuple is taken from the first list, the  second  element  is  taken
              from  the  corresponding element in the second list, and the third element is taken
              from the corresponding element in the third list.

       zipwith(Combine, List1, List2) -> List3

              Types:

                 Combine = fun((X, Y) -> T)
                 List1 = [X]
                 List2 = [Y]
                 List3 = [T]
                 X = Y = T = term()

              Combines the elements of two lists of equal length into one list. For each pair  X,
              Y of list elements from the two lists, the element in the result list is Combine(X,
              Y).

              zipwith(fun(X, Y) -> {X,Y} end, List1, List2) is equivalent to zip(List1, List2).

              Example:

              > lists:zipwith(fun(X, Y) -> X+Y end, [1,2,3], [4,5,6]).
              [5,7,9]

       zipwith3(Combine, List1, List2, List3) -> List4

              Types:

                 Combine = fun((X, Y, Z) -> T)
                 List1 = [X]
                 List2 = [Y]
                 List3 = [Z]
                 List4 = [T]
                 X = Y = Z = T = term()

              Combines the elements of three lists of equal length into one list. For each triple
              X,  Y,  Z  of list elements from the three lists, the element in the result list is
              Combine(X, Y, Z).

              zipwith3(fun(X, Y, Z) ->  {X,Y,Z}  end,  List1,  List2,  List3)  is  equivalent  to
              zip3(List1, List2, List3).

              Examples:

              > lists:zipwith3(fun(X, Y, Z) -> X+Y+Z end, [1,2,3], [4,5,6], [7,8,9]).
              [12,15,18]
              > lists:zipwith3(fun(X, Y, Z) -> [X,Y,Z] end, [a,b,c], [x,y,z], [1,2,3]).
              [[a,x,1],[b,y,2],[c,z,3]]