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