swap12([X, Y|Tail], [Y, X|Tail]).
test_answer :-
swap12([a, b, c, d], L),
writeln(L).
test_answer2 :-
\+ swap12(L, [1]),
writeln(‘OK’).
test_answer3 :-
swap12(L, [b, a]),
writeln(L).
test_answer4 :-
swap12(L1, L2),
writeln(‘OK’).
%swap12(?List1, ?List2) = true if List1 == List2, except List1[0] & List2[0] are swapped. Always fails with lists with length < 2