member/2 /MINERVA/Manual/Reference/Predicates/lists/member/

member/2

tests whether a term is member of a list.

member/2 declares, that Term is a member of List. Different cases can be:

If Term and List are not variables then member/2 tests whether Term is member of List. By backtracking member/2 succeeds as often as Term is member of List.

If Term is a variable and List is bound to a list then member/2 retrieves (by backtracking) all elements of List and unifies them with Term.
If Term is not a variable and List is an open list (i.e. the tail of the list is a variable) then member/2 asserts that Term is a member of List.

Term                   term
List                   list

member(b,[a,b,c]).	Succeeds.
member(X,[a,b]).	Succeeds twice with substitution X <- a X <- b
member(a,X).	Succeeds infinitely with substitution X <- [a\|_1], X <- [_2,a\|_3], X <- [_4,_5,a\|_6],....
member(X,Y).	Succeeds infinitely with substitution X <- _1, Y <- [_1\|_2], X <- _3, Y <- [_4,_3\|_5], X <- _6, Y <- [_7,_8,_6\|_9], ...

This predicate is not part of the ISO-Prolog Standard.