Function pm#relation_from_collection_to_number (pm#collection -> sumo#number)
  supertype:  pm#relation_from_collection  pm#how_much_relation
  subtype:  pm#size__number_of_elements (pm#collection -> sumo#nonnegative_integer)
     subtype:  kif#length (sumo#list -> sumo#nonnegative_integer)
  subtype:  pm#minimal_size__minimalsize (pm#collection -> sumo#nonnegative_integer)
  subtype:  pm#maximal_size__maximalsize (pm#collection -> sumo#nonnegative_integer)
  subtype:  pm#percentage (pm#collection -> sumo#number)
  subtype:  pm#average (pm#number_container -> sumo#number)  to specify an average on the values