Relation pm#symmetric_relation__symmetricrelation (?,?)  this category only serves structuration purposes: it is instance of pm#symmetric_relation_type which is not instance of pm#class_of_inheritable_relation_type
  subtype:  equivalence_relation  different  independent_probability  overlaps_temporally  connected  meets_spatially  overlaps_spatially  overlaps_partially  connected_engineering_components  sibling  legal_relation  spouse  consistent
  type:  symmetric_relation_type  when (?REL ?INST1 ?INST2) implies (?REL ?INST2 ?INST1), for all ?INST1 and ?INST2
  supertype:  binary_relation_with_particular_mathematical_property (?,?)
     supertype:  relation_with_particular_mathematical_property (*)
        supertype:  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:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type