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