sumo#inverse (pm#binary_relation_type -> pm#binary_relation_type)  for inverseOf(R,S) read: R is the inverse of S; i.e. if R(x,y) then S(y,x) and vice versa; in WebKB, use the link '-'
  equal:  pm#inverse (pm)  owl#inverse_of (pm)
  type:  pm#binary_predicate_type  the class of predicates relating two items - its valence is two
  type:  pm#irreflexive_relation_type  r is irreflexive if r(?i,?i) holds for no value of ?i
  type:  pm#symmetric_relation_type  when (?REL ?INST1 ?INST2) implies (?REL ?INST2 ?INST1), for all ?INST1 and ?INST2
  type:  pm#intransitive_relation_type  a binary_relation ?REL is intransitive only if (?REL ?INST1 ?INST2) and (?REL ?INST2 ?INST3) imply not (?REL ?INST1 ?INST3), for all ?INST1, ?INST2, and ?INST3
  supertype:  pm#relation_to_another_binary_relation_type (pm#binary_relation_type,pm#binary_relation_type)
     supertype:  pm#relation_from_binary_relation_type (pm#binary_relation_type,*)
        supertype:  pm#relation_from_relation_type (pm#relation_type,*)
           supertype:  pm#relation_from_type (pm#type,*)  type of relations from a concept/relation type, i.e. in RDFS terminology, from a class or a property
              supertype:  pm#relation_from_collection (pm#collection,*)
                 supertype:  pm#relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
                    >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
                 supertype:  pm#what_relation (*)
                    supertype:  pm#wh-/how_relation (*)  this type permits to categorize relations according to the usual who/what/why/where/when/how questions ; this is a traditional but very subjective and ineffective way of categorizing relations 
                       >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  pm#irreflexive_relation__irreflexiverelation (?,?)  this category only serves structuration purposes: it is instance of pm#irreflexive_relation_type which is not instance of pm#class_of_inheritable_relation_type
     supertype:  pm#binary_relation_with_particular_mathematical_property (?,?)
        supertype:  pm#relation_with_particular_mathematical_property (*)
           supertype:  pm#relation_with_particular_property (*)  this rather fuzzy type permits to group categorization schemes less common than those covered by the previous sibling categories
              >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  pm#intransitive_relation (?,?)  this category only serves structuration purposes: it is instance of pm#intransitive_relation_type which is not instance of pm#class_of_inheritable_relation_type
     supertype:  pm#binary_relation_with_particular_mathematical_property (?,?)