Relation pm#specializing_type (pm#type,?)
  supertype:  pm#relation_from_type  pm#partial_ordering_relation
  instance of:  pm#partial_ordering_relation_type
  inverse:  pm#generalizing_type
  subtype:  pm#instance (pm#type,?)  the ':' link in the FT notation
  subtype:  pm#subtype__subtype_or_equal (pm#type,pm#type)  subtype links should actually be strict subtype links or not much checking can be done
     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