Relation pm#subclass_of_or_equal (pm#set_or_class,pm#set_or_class)
  supertype:  pm#relation_to_another_set_or_class  this category is needed to group SUMO relations between classes which cannot be subtype of pm#relation_from_type because their signatures curiously also involve sets
  subtype:  sumo#subclass__subclass_of (sumo#set_or_class,sumo#set_or_class)  if the common reading conventions of parameters had been respected, this type would have been named subclass_of; every instance of the 1st argument is also an instance of the 2nd argument; a class may have multiple superclasses and subclasses
     subtype:  sumo#immediate_subclass__immediate_subclass_of (sumo#set_or_class,sumo#set_or_class)  the 1st argument is a subclass of the 2nd argument and there is no other subclass of the 2nd argument such that the 1st is also a subclass of the 2nd; in WebKB, use the link '<'
  subtype:  rdfs#sub_class_of__subclassof__super_class__superclas (rdfs#class,rdfs#class)  in WebKB, use the link '<'