This function returns the OID of the first element within the text or file entity specified by OID oid. This function returns the null OID if oid is not a file or text entity, or if there are no elements within the entity. If oid is a file entity reference, the OID returned would be the top of a different document tree than the one oid is in. If oid is a text entity, the OID returned is in the same doc as the one oid is in.