NULL the current engine associated with the calling
thread is stored at the given location. If engine equals
PL_ENGINE_MAIN the initial engine is attached to the
calling thread. If engine is PL_ENGINE_CURRENT
the engine is not changed. This can be used to query the current engine.
This call returns
PL_ENGINE_SET if the engine was switched successfully,
PL_ENGINE_INVAL if engine is not a valid engine
handle and
PL_ENGINE_INUSE if the engine is currently in use by
another thread.
Engines can be changed at any time. For example, it is allowed to
select an engine to initiate a Prolog goal, detach it and at a later
moment execute the goal from another thread. Note, however, that the
term_t, qid_t and fid_t types are
interpreted relative to the engine for which they are created. Behaviour
when passing one of these types from one engine to another is undefined.
In the single-threaded version this call only succeeds if engine refers to the main engine.