def METHOD_NAME(self):
    """
    Enter a new scope within the namespace.
    Called as a context manager, e.g. `with namespace.enter_scope():`
    All items that are added within the context are removed upon exit.
    """
    # NOTE cyclic imports!
    from vyper.semantics import environment
    self._scopes.append(set())
    if len(self._scopes) == 1:
        # add mutable vars (`self`) to the initial scope
        self.update(environment.get_mutable_vars())
    return self