Cleaning up sorting of error messages.

    def add(self, location, fmt, *args):

        """Append error unilaterally."""

        if isinstance(location, type(None)):

            coords = ''

        elif isinstance(location, str):

            coords = '{0}: '.format(location)

        elif isinstance(location, tuple):

            filename, line_number = location

            coords = '{0}:{1}: '.format(*location)

        else:

            assert False, 'Unknown location "{0}"/{1}'.format(location, type(location))

        self.messages.append(coords + fmt.format(*args))

        self.messages.append((location, fmt.format(*args)))

    def report(self, stream=sys.stdout):

        """Report all messages."""

        """Report all messages in order."""

        if not self.messages:

            return

        for m in sorted(self.messages):

            print(m, file=stream)

        def pretty(item):

            location, message = item

            if isinstance(location, type(None)):

                return message

            elif isinstance(location, str):

                return location + ': ' + message

            elif isinstance(location, tuple):

                return '{0}:{1}: '.format(*location) + message

            else:

                assert False, 'Unknown item "{0}"'.format(item)

        def key(item):

            location, message = item

            if isinstance(location, type(None)):

                return ('', -1, message)

            elif isinstance(location, str):

                return (location, -1, message)

            elif isinstance(location, tuple):

                return (location[0], location[1], message)

            else:

                assert False, 'Unknown item "{0}"'.format(item)

        for m in sorted(self.messages, key=key):

            print(pretty(m), file=stream)

def read_markdown(parser, path):