% member(X, L) is true iff X is a member of list L.

member(X, [X | _]).		    % X is the first element

member(X, [_ | L]) :- member(X, L). % X is a member of the remaining elements

/* Examples:

| ?- member(a, [b, c, a, d]).

yes
| ?- member(e, [b, c, a, d]).

no
| ?- member(X, [b, c, a, d]).

X = b ;

X = c ;

X = a ;

X = d ;

no

*/