code: def creating_subvariant (self, s = None):
          
          assert s is None or isinstance(s, Subvariant)
          if s and not self.creating_subvariant ():
              if self.creating_subvariant ():
                  raise BaseException ("Attempt to change 'dg'")
  
              else:
                  self.creating_subvariant_ = s
  
          return self.creating_subvariant_
docs: Gets or sets the subvariant which created this target. Subvariant
          is set when target is brought into existance, and is never changed
          after that. In particual, if target is shared by subvariant, only
          the first is stored.
          s:  If specified, specified the value to set,
                  which should be instance of 'subvariant' class.