Render a parent block.

def super(self, name, current):
        """Render a parent block."""
        try:
            blocks = self.blocks[name]
            index = blocks.index(current) + 1
            blocks[index]
        except LookupError:
            return self.environment.undefined('there is no parent block '
                                              'called %r.' % name,
                                              name='super')
        return BlockReference(name, self, blocks, index)