% Define a predicate `elem(E,L)` which is true if the first argument E
% is contained in the second list argument L.

% If E is the first element of L, then elem(E,L) holds.
%elem(E,[E|_]).
% If E is the second element of L, then elem(E,L) holds.
%elem(E,[X,E|_]).
% If E is the third element of L, then elem(E,L) holds.
% ...

% If E is the first element of L, then elem(E,L) holds.
elem(E,[E|_]).
% If E is an element of the rest of L, then elem(E,L) holds.
elem(E,[_|R]) :- elem(E,R).