Relation pm#why_relation__whyrelation (*)
  subtype:  pm#cause  pm#consequence  pm#method  pm#goal  pm#triggering_event  pm#ending_event  pm#precondition  pm#postcondition  pm#purpose
  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 
     supertype:  pm#relation__related_thing__relatedthing___related_with (*)  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type


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