Function rdf#predicate (rdf#statement -> pm#binary_relation_type)  the property used in a statement when representing the statement in reified form
  supertype:  pm#rdf_reification_relation__rdfreificationrelation (?,?)
     supertype:  pm#attributive_relation__attributiverelation (*)  like pm#binary_relation, this type mostly exists to categorize what cannot be categorized elsewhere
        supertype:  pm#relation_playing_a_special_role (*)  this type permits to categorize relations according to their roles ; this is a traditional but quite subjective 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