The [NOT] MEMBER [OF] collection member expression determines whether a value is a member of a collection. The value and the collection members must have the same type.
If either the collection-valued or single-valued path expression is unknown, then the collection member expression is unknown. If the collection-valued path expression designates an empty collection, then the collection member expression is FALSE.
The OF keyword is optional.
The following example tests whether a line item is part of an order:
SELECT o FROM Order o WHERE :lineItem MEMBER OF o.lineItems