Relation pm#relation_to_collection (*,collection)
  exclusion:  relation_to_situation  spatial_relation_to_entity_with_spatial_feature  relation_to_time  relation_to_attribute_or_quality_or_measure
  subtype:  member_of  list_of  set_of  parts  item  in_list  list_fn  function_returning_a_collection_from_2_elements  relation_between_collections  relation_to_set_or_class  relation_to_type  relation_to_string
  supertype:  relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
     >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  what_relation (*)
     supertype:  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 
        >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type