Flattening only one level of a list in Prolog

Question

I\'m working on a problem to flatten only one level of a list in Prolog. For example, [[1],[2,3]] would become [1,2,3], but [[1,[2]],3]

Answer 1

You need 3 simple clauses, I will show just the most complex one

flat([H|T],R) :- is_list(H), flat(T,T1), append(H,T1,R).

other two clauses are the base recursion case, and a copy as is of head to result.

You should also place a cut in the clause I've shown, otherwise on backtracking you'll get wrong results (due to firing of the clause copy as is)