Relation sumo#periodical_issue_fn ()
  type:  pm#binary_function_type  class of functions requiring two arguments
  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