Individual pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  >part:  relation_from/to_thing_of_common_kind  relation_playing_a_special_role  wh-/how_relation  relation_with_particular_property
  type:  relation_type  there are three kinds of relation(_types): pm#predicate_type, pm#function_type and sumo#list; both predicates and functions denote sets of ordered n-tuples; the difference between these two classes is that predicates cover formula-forming operators, while functions cover term-forming operators; a list, on the other hand, is a particular ordered n-tuple

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