Render a 'true' or (if available) 'false' block based on a
        boolean.

def render(self, namespace):
        '''Render a 'true' or (if available) 'false' block based on a
        boolean.'''
        if (self._isbool and
            namespace[self._evaluate]) or \
            (not self._isbool and
             namespace[self._evaluate](*[namespace[arg]
                                         for arg in self._args])):
            return self._block_true.render(namespace)
        if self._block_false is not None:
            return self._block_false.render(namespace)
        return None