memberchk(E, L) succeeds if E is a member of the list L. No attempt is made to retry the goal during backtracking - so if E appears multiple times in L only the first occurrence will be matched.
Examples
?- memberchk(a, [a,b,c]).
yes
?- memberchk(b, [a,b,c]).
yes
?- memberchk(c, [a,b,c]).
yes
?- memberchk(d, [a,b,c]).
no
?- memberchk(d, []).
no
?- memberchk(X, []).
no
?- memberchk([], []).
no
?- memberchk(X, [a,b,c|d]).
X = a
yes
?- memberchk(a, [a,b,c|d]).
yes
?- memberchk(b, [a,b,c|d]).
yes
?- memberchk(c, [a,b,c|d]).
yes
?- memberchk(d, [a,b,c|d]).
Expected empty list or variable but got: ATOM with value: d
?- memberchk(z, [a,b,c|d]).
Expected empty list or variable but got: ATOM with value: d
?- memberchk(X, [a,b,c]).
X = a
yes
?- memberchk(p(X,b), [p(a,b), p(z,Y), p(x(Y), Y)]).
X = a
Y = UNINSTANTIATED VARIABLE
yes
?- memberchk(p(a,X),[p(x,y),b,p(Y,Y)]).
X = a
Y = a
yes
?- memberchk(a, X).
X = [a|_]
yes
?- memberchk(p(a,X),a).
Expected list or empty list but got: ATOM with value: a
?- memberchk(something, [something|_]).
yes
?- memberchk(anything, [something|_]).
yes
?- memberchk(anything, [something|X]).
X = [anything|_]
yes