
merge(+Key, +Order, +List1, +List2, ?List3)

   Succeeds if List3 is a merged list of List1 and List2.  If both lists are
sorted, List3 will be sorted.  The sort is done according to the Key and
Order specifications.



Arguments
   +Key                A non-negative integer or a list of positive integers.
   +Order              One of the atoms =<, >=, < or >.
   +List1              List.
   +List2              List.
   ?List3              List or variable.

Type
   Comparing and Sorting

Description
   Used to merge the sorted lists List1 and List2 to give the sorted list
   List3.


   If List1 and List2 are not lists of compound terms, use Key = 0.


   If List1 and List2 are lists of compound terms, then the sort will be
   according to the Keyth argument of the lists' elements.  If Key = 0,
   then the sort is on the entire terms.


   For two lists [e1,e2,e3] and [f1,f2,f3], e1 is compared to f1.  The
   resulting element (dictated by Key, Order and the standard ordering
   below) is put into List3, and the remaining element is compared to the
   next element in the other list.  This process continues until both lists
   are exhausted.


   In particular, this will merge two sorted lists into a sorted list.


   In all cases where List1 and List2 are sorted, Order specifies whether
   the lists are sorted into ascending (<, =<) or descending (>, >=) order
   and whether duplicates are to be retained (=<, >=) or eliminated (<, >).
   The way to remember the Order argument is that it is the relation which
   holds between adjacent elements in the result.


   The sort is done according to the standard ordering of terms.
   See compare/3 for the definition of this standard ordering.




Resatisfiable
      No.

Fail Conditions
      Fails if List3 does not unify with the merging of the lists List1 and
   List2 outlined above.



Exceptions
     5 --- Key is greater than 0, and one of List1 and List2 does not    have all elements compound terms.
     5 --- Key is not an integer or a list of integers.
     6 --- One of the compound terms in List1 or List2 has not got as    many as Key arguments.

Examples
   
Success:
      merge(0,<,[2,4,6],[1,3,5],L).
                      (gives L=[1,2,3,4,5,6]).
      merge(0,<,[f(1),f(7)],[f(8),f(10)],L).
                      (gives L=[f(1),f(7),f(8),f(10)]).
      merge(0,<,[f(2),f(1)],[f(3),f(8)],L).
                      (gives L=[f(2),f(1),f(3),f(8)]).
      merge(0,<,[f(2)],[f(6),f(1)],L).
                      (gives L=[f(2),f(6),f(1)]).
      merge(0,>,[1,e,q],[Q,2,a],L).
                      (gives Q=_g60,L=[_g60,1,2,a,e,q]).
      merge(0,>,[f(8),f(6)],[f(4),f(1)],L).
                      (gives L=[f(8),f(6),f(4),f(1)]).
      merge(2,<,[f(2,1),f(6,4)],[f(6,3),f(8,6)],L).
                      (gives L=[f(2,1),f(6,3),f(6,4),f(8,6)]).
      merge(2,<,[q(2,1),f(6,4)],[a(6,3),i(8,6)],L).
                      (gives L=[q(2,1),a(6,3),f(6,4),i(8,6)]).
      merge(2,<,[f(a,b),f(c,a)],[f(k,a)],L).
                      (gives L=[f(k,a),f(a,b),f(c,a)).
      merge(0,=<,[1,2],[3,4,4,5],L).
                      (gives L=[1,2,3,4,4,5]).
      merge([2,1], =<, [f(1,a(1)), f(0,a(3))], [f(3,a(2)), f(1,a(4))], L).
                      (gives L=[f(1,a(1)), f(3,a(2)), f(0,a(3)), f(1,a(4))]).
Fail:
      merge(0,<,[2,4,6],[1,3,5],[1,2,3,4,5]).
Error:
      merge(1,<,[f(1,2),f],[f(3,4),h(1,2)],L). (Error 5).
      merge(0.0,<,[f(1)],[f(2)],L).            (Error 5).
      merge(2,<,[f(1,2)],[f(8)],L).            (Error 6).





See Also
   merge / 3, compare / 3
