%% We need two base clauses for stopping the recursion. This one is %% reached when the list does not contain any occurrences of the %% element that is supposed to be removed. In this case, the empty %% list is reached at some point. And the empty list is returned as %% result. remove(_,,). %% If the element which is supposed to be removed is the head of the %% list, we remove the head. Hence, the result is the tail. remove(X,[X|Tail],Tail). %% If the head does not match the specified term X, try to remove an %% element matching X from the tail of the list. The output of the %% predicate is constructed by sticking the head back onto OutTail %% (which is the result of removing X from the tail of the input list). remove(X,[Head|InTail],[Head|OutTail]) :- remove(X,InTail,OutTail).