Call a block and register it for the template.

def visit_Block(self, node, frame):
        """Call a block and register it for the template."""
        level = 1
        if frame.toplevel:
            # if we know that we are a child template, there is no need to
            # check if we are one
            if self.has_known_extends:
                return
            if self.extends_so_far > 0:
                self.writeline('if parent_template is None:')
                self.indent()
                level += 1
        context = node.scoped and 'context.derived(locals())' or 'context'
        self.writeline('for event in context.blocks[%r][0](%s):' % (
                       node.name, context), node)
        self.indent()
        self.simple_write('event', frame)
        self.outdent(level)