Enumerates, by backtracking, all existing engines. Engine names shall be regarded as opaque terms; users shall not rely on its type.
Modes and number of proofs
threaded_engine(?nonvar) - zero_or_more
% check that the worker_1 engine exists: | ?- threaded_engine(worker_1). % write the names of all existing engines: | ?- forall( threaded_engine(Engine), (writeq(Engine), nl) ).