rd arguments are valid, but their actual semantics
are yet to be decided.  See PEP 612 for details.

Parameter specification variables can be introspected. e.g.:

   P.__name__ == 'T'
   P.__bound__ == None
   P.__covariant__ == False
   P.__contravariant__ == False

Note that only parameter specification variables defined in global scope can
be pickled.
c