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