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:  pm#equivalence_relation  pm#different  sumo#independent_probability  sumo#overlaps_temporally  sumo#connected  sumo#meets_spatially  sumo#overlaps_spatially  sumo#overlaps_partially  sumo#connected_engineering_components  sumo#sibling  sumo#legal_relation  sumo#spouse  sumo#consistent
  type:  pm#symmetric_relation_type  when (?REL ?INST1 ?INST2) implies (?REL ?INST2 ?INST1), for all ?INST1 and ?INST2
  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