docs: def semantic_alert(visitor, block):
      """
      Format:
          
          {% alert class=error %}
          message
          {% endalert %}
      """
      txt = []
      cls = block['kwargs'].get('class', '')
      txt.append('<div class="ui %s message">' % cls)
      text = visitor.parse_text(block['body'], 'article')
      txt.append(text)
      txt.append('</div>')
      return '\n'.join(txt)