Return the number of alternatives as indicated by the database
internal hashed indexing. This is a rough measure for the number
of alternatives we can expect for an rdf_has/3 call using the
given three arguments. When called with three variables, the
total number of triples is returned. This estimate is used in
query optimisation. See also rdf_predicate_property/2 and
rdf_statistics/1 for additional information to help optimizers.
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