As rdf_insert_literal_map/3.
In addition, if Key is a new key in
Map, unify KeyCount with the number of keys in Map.
This serves two purposes. Derived maps, such as the stem and metaphone
maps need to know about new keys and it avoids additional foreign calls
for doing the progress in rdf_litindex.pl.
rdf_insert_literal_map(+Map,
+Key, +Value, -KeyCount)