code: def fill_attrs(self, attrs):
          
          for trname, attrname in self.transitions_at.items():
  
              implem = self.implementations[trname]
  
              if attrname in attrs:
                  conflicting = attrs[attrname]
                  if not self._may_override(implem, conflicting):
                      raise ValueError(
                          "Can't override transition implementation %s=%r with %r" %
                          (attrname, conflicting, implem))
  
              attrs[attrname] = implem
          return attrs
docs: Update the 'attrs' dict with generated ImplementationProperty.