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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 sub-lists of ListOfLists have been appended. For
              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. For 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.

              > 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.

       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 which contains N copies of the term Elem. For 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/2 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 the 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. For 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. For 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 would usually be preferred to foldr/3.

       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 prior to 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. The function  lists:keyfind/3
              (introduced in R13A) is in most cases 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 the 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 is implementation dependent.

       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()

              mapfoldl combines the operations of map/2 and foldl/3 into one  pass.  An  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()

              mapfoldr 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  sub-lists  of  ListOfLists.  All
              sub-lists  must  be  sorted  prior  to  evaluating this function. When two elements
              compare  equal,  the  element  from  the  sub-list  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 prior to 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  prior  to  evaluating  this
              function. Fun(A, B) should return true if A compares less than or equal to B in the
              ordering, false otherwise. 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 prior to 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. For 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. For 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]}

              See also splitwith/2 for a different way to partition a list.

       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 the tail Tail
              appended. For example:

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

       seq(From, To) -> Seq

       seq(From, To, Incr) -> Seq

              Types:

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

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

              Failure: If To<From-Incr and Incr is positive,  or  if  To>From-Incr  and  Incr  is
              negative, or 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) should return true if A compares less than or equal to B in
              the ordering, false otherwise.

       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 rest of the 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]}

              See also partition/2 for a different way to partition a list.

       sublist(List1, Len) -> List2

              Types:

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

              Returns  the  sub-list of List1 starting at position 1 and with (max) 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 sub-list of List1 starting at Start and with (max) Len elements. It is
              not an error for Start+Len to exceed the length of the list.

              > 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 which is a copy  of  List1,  subjected  to  the  following
              procedure: for each element in List2, its first occurrence in List1 is deleted. For
              example:

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

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

          Warning:
              The complexity of lists:subtract(A,  B)  is  proportional  to  length(A)*length(B),
              meaning  that  it  will be very slow if both A and B are long lists. (Using ordered
              lists and ordsets:subtract/2 is a much better choice if both lists are long.)

       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 prior to evaluating this function.  When  two  tuples
              compare  equal,  the  tuple  from  TupleList1 is picked and the one from TupleList2
              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 the list TupleList1 where all  but
              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  sub-lists  of  ListOfLists.  All
              sub-lists  must  be  sorted  and  contain  no  duplicates  prior to evaluating this
              function. When two elements compare equal, the element from the sub-list  with  the
              lowest position in ListOfLists is picked and the other one 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 prior to evaluating  this  function.  When
              two elements compare equal, the element from List1 is picked and the one from List2
              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
              prior to evaluating this function. Fun(A, B) should return true if A compares  less
              than  or  equal  to  B  in the ordering, false otherwise. When two elements compare
              equal, the element from List1 is picked and the one from List2 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 prior to 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
              one 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  but  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 which contains the sorted elements of List1 where all but the first
              element of the elements comparing equal according to the ordering function Fun have
              been deleted. Fun(A, B) should return true if A compares less than or equal to B in
              the ordering, false otherwise.

       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 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 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()

              Combine 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  will  be
              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()

              Combine  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  will
              be 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]]