Calls the extender.

def visit_Extends(self, node, frame):
        """Calls the extender."""
        if not frame.toplevel:
            self.fail('cannot use extend from a non top-level scope',
                      node.lineno)

        # if the number of extends statements in general is zero so
        # far, we don't have to add a check if something extended
        # the template before this one.
        if self.extends_so_far > 0:

            # if we have a known extends we just add a template runtime
            # error into the generated code.  We could catch that at compile
            # time too, but i welcome it not to confuse users by throwing the
            # same error at different times just "because we can".
            if not self.has_known_extends:
                self.writeline('if parent_template is not None:')
                self.indent()
            self.writeline('raise TemplateRuntimeError(%r)' %
                           'extended multiple times')

            # if we have a known extends already we don't need that code here
            # as we know that the template execution will end here.
            if self.has_known_extends:
                raise CompilerExit()
            else:
                self.outdent()

        self.writeline('parent_template = environment.get_template(', node)
        self.visit(node.template, frame)
        self.write(', %r)' % self.name)
        self.writeline('for name, parent_block in parent_template.'
                       'blocks.%s():' % dict_item_iter)
        self.indent()
        self.writeline('context.blocks.setdefault(name, []).'
                       'append(parent_block)')
        self.outdent()

        # if this extends statement was in the root level we can take
        # advantage of that information and simplify the generated code
        # in the top level from this point onwards
        if frame.rootlevel:
            self.has_known_extends = True

        # and now we have one more
        self.extends_so_far += 1