def shall_save(self, form, name, composite_form):
if composite_form.empty_permitted and not composite_form.has_changed():<EOL><INDENT>return False<EOL><DEDENT>return True<EOL>
Return ``True`` if the given ``composite_form`` (the nested form of
this field) shall be saved. Return ``False`` if the form shall not be
saved together with the super-form.

By default it will return ``False`` if the form was not changed and the
``empty_permitted`` argument for the form was set to ``True``. That way
you can allow empty forms.