Of these three predicates, rdf_load_file/7 does the real work.
The others deal with the possibility that the graph is being
loaded by another thread. In that case, we wait for the other
thread to complete the work.
See also
- Code is modelled closely after how concurrent loading
is handled in SWI-Prolog's boot/init.pl
To be done
- What if both threads disagree on what is loaded into the
graph?