% Predicate member(E,L) <=> E is an element of list L

% E is the first element in the list:
member(E,[E|_]).
% E is the second element in the list:
%member(E,[_,E|_]).
% E is the third element in the list:
%member(E,[_,_,E|_]).
% ....

% If E is an element the rest of a list, then it is an element of the list
member(E,[_|R]) :- member(E,R).