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