code: def _tailCallback(f, uid):
      
      def t(*args):
          raise _TailCall(f, args, uid)
      t.C = f
      return t
docs: This is the "callable" version of the continuation, which sould only
      be accessible from the inside of the function to be continued. An
      attribute called "C" can be used in order to get back the public
      version of the continuation (for passing the continuation to another
      function).