Allowed more easily subclassing of BlockNode tags.

                if block is None:

                    block = self

                # Create new block so we can store context without thread-safety issues.

                block = BlockNode(block.name, block.nodelist)

                block = type(self)(block.name, block.nodelist)

                block.context = context

                context['block'] = block

                result = block.nodelist.render(context)