Relation sumo#relative_complement_fn ()
  type:  pm#binary_function_type  class of functions requiring two arguments
  type:  pm#total_valued_relation_type  when there exists an assignment for the last argument position of the relation given any assignment of values to every argument position except the last one; note that declaring a relation to be both a total_valued_relation and a single_valued_relation means that it is a total function