Availability:
:- use_module(library(semweb/rdf11)).
[semidet]rdf_is_subject(@Term)Only blank nodes and IRIs can appear in the subject position.
Success of this goal does not imply that the subject term is present in the database (see rdf_subject/1) for that).
Since blank nodes are represented by atoms that start with `_:` and
an IRIs are atoms as well, this is equivalent to
atom(Term).