SWI-Prolog -- blob

Availability:built-in

blob_released(@Term)

True if Term is a blob whose data was released using PL_free_blob(). Such a blob still exists as a term and keeps its type, but the object it referred to is gone: it writes as <Type>(freed) and every predicate that expects the real thing rejects it. Note that only blobs can be released; an ordinary atom is reclaimed as a whole by the atom garbage collector and remains valid for as long as it exists.