Language: Prolog

Implement a Prolog swap predicate in the form swap(Before, After) . The list After should be the list Before with the first and second halves swapped. If the list has an odd number of elements, the middle element should remain in place. At least one of the variables should be instantiated in the query. Examples of queries and expected results for each case are given below:

Case 1: Before and After both instantiated

?- swap([1,2,3,4,5], [4,5,3,1,2]).

true.

Case 2: Before instantiated only

?- swap([1,2,3,4,5], R).

R = [4,5,3,1,2].

Case 3: After instantiated only

?- swap(L, [1,2,3,4,5,6]).

L = [4,5,6,1,2,3].

If neither variable is instantiated an exception will be thrown.