Relation pm#strict_subtype (type,type) the '>' link in the FT notation
supertype: subtype__subtype_or_equal subtype links should actually be strict subtype links or not much checking can be done
equal: properly_subsumes (pm)
subtype: properly_subsumes_leaf__PSBL (type,type) the 2nd type is a leaf type properly subsumed by the 1st type