% member(E,L) <=> Is E an element of list L?
member(E,[E|_]).
%member(E,[_,E|_]).
%member(E,[_,_,E|_]).
%member(E,[_,_,_,E|_]).
%...

% if E is in the rest list, then it is in a list:
member(E,[_|R]) :- member(E,R).