code: def visit_Block(self, node, frame):
          
          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)
docs: Call a block and register it for the template.