Solution

%% The empty list is already flat.
myflatten([],[]).

%% For a non-empty list: flatten the head and flatten the tail and
%% append the results.
myflatten([Head|InTail],Out) :-
	myflatten(Head,FlatHead),
	myflatten(InTail,OutTail),
	append(FlatHead,OutTail,Out).
%% When trying to flatten the head, you might find that the head is
%% not itself a list. In this case the head of the input list simply
%% becomes the head of the output list. 
myflatten([Head|InTail], [Head|OutTail]) :-
	Head \= [],
	Head \= [_|_],
        myflatten(InTail,OutTail).
    

Back to the exercise.