code: def find_shadowed(self, extra=()):
          
          i = self.identifiers
          return (i.declared | i.outer_undeclared) & \
                 (i.declared_locally | i.declared_parameter) | \
                 set(x for x in extra if i.is_declared(x))
docs: Find all the shadowed names.  extra is an iterable of variables
          that may be defined with `add_special` which may occour scoped.