Relation sumo#mereological_difference_fn (sumo#object,*)
  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
  type:  pm#spatial_relation_type  the class of relations that are spatial in a wide sense, e.g., mereological relations and topological relation
  supertype:  pm#spatial_relation_from_entity_with_spatial_feature (sumo#object,*)
     supertype:  pm#relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
        >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type