Used to test whether Term is a handle. Handles are references to anonymous ECLiPSe objects (such as streams, engines, bags, records, shelves, stores) or to external data created via the foreign language interfaces.
    ?- bag_create(B), is_handle(B).
    B = $&(bag,"371bnb")
    Yes (0.00s cpu)
    ?- is_handle(_).
    No (0.00s cpu)
    ?- is_handle(42).
    No (0.00s cpu)