Relation pm#injective_binary_relation (?,?)  if P is injective, then if P(x,y) and P(z,y) then x=z; e.g., if nameOfMonth(m,"Feb") and nameOfMonth(n,"Feb") then m and n are the same month; this category only serves structuration purposes: it is instance of pm#injective_binary_relation_type which is not instance of pm#class_of_inheritable_relation_type
  type:  injective_binary_relation_type  if P is injective, then if P(x,y) and P(z,y) then x=z; e.g., if nameOfMonth(m,"Feb") and nameOfMonth(n,"Feb") then m and n are the same month
  supertype:  binary_relation_with_particular_mathematical_property (?,?)
     supertype:  relation_with_particular_mathematical_property (*)
        supertype:  relation_with_particular_property (*)  this rather fuzzy type permits to group categorization schemes less common than those covered by the previous sibling categories
           >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type