def known_indent(self, match, context, next_state):
        """
        Handle a known-indent text block. Extend or override in subclasses.

        Recursively run the registered state machine for known-indent indented
        blocks (`self.known_indent_sm`). The indent is the length of the
        match, ``match.end()``.
        """
        indented, line_offset, blank_finish = \
              self.state_machine.get_known_indented(match.end())
        sm = self.known_indent_sm(debug=self.debug,
                                 **self.known_indent_sm_kwargs)
        results = sm.run(indented, input_offset=line_offset)
        return context, next_state, results