Function pm#function_returning_a_collection_from_2_elements (?,? -> pm#collection)
  supertype:  pm#relation_to_collection
  subtype:  kif#cons (?,sumo#list -> sumo#list)
  subtype:  kif#adjoin (?,sumo#list -> sumo#list)
  subtype:  kif#remove (?,sumo#list -> sumo#list)