Given a set, list, or function s, the membertype(x, s) function determines if there is an operand of type x.  Given a module, it determines if there is an assigned export of type x. It returns true if so, and false otherwise.

If a third argument 'p' is present and membertype yields true, then the position of the first x in s is assigned to p in the case of sets and lists, while the member instance is assigned to the name p if s is a module.