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