<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _writable_required(self, path):
 # type: (Text) -> FS """Check that ``path`` is writeable. """

        if self.write_fs is None:
            raise errors.ResourceReadOnly(path)
        return self.write_fs