Relation sumo#greatest_common_divisor_fn ()
  type:  pm#function_type  term-forming relation that maps from a n-tuple of arguments to a range and that associates this n-tuple with at most one range element; note that the range is a set_or_class, and each element of the range is an instance of the set_or_class
  type:  pm#partial_valued_relation_type  relation type that is not a total_valued_relation_type, i.e. just in case assigning values to every argument position except the last one does not necessarily mean that there is a value assignment for the last argument position; note that, if a sumo#relation is both a partial_valued_relation and a single_valued_relation, then it is a partial function
  type:  pm#variable_arity_relation_type  class of relations that do not have a fixed number of arguments