def render(self, engine, context):
        """
        Returns: a string of type unicode.

        """
        # We avoid use of the ternary operator for Python 2.4 support.
        def get_unicode(node):
            if type(node) is unicode:
                return node
            return node.render(engine, context)
        parts = map(get_unicode, self._parse_tree)
        s = ''.join(parts)

        return unicode(s)
Returns: a string of type unicode.