code: def add_label(self, name, color):
          
          color_upper = color.upper()
          if not self._color_re.match(color_upper):
              raise ValueError('Invalid color: {}'.format(color))
  
          labels_tag = self.root[0]
          last_id = int(labels_tag[-1].get('id'))
          new_id = str(last_id + 1)
  
          new_label = etree.Element('label', id=new_id, color=color_upper)
          new_label.text = name
  
          labels_tag.append(new_label)
docs: Add a new label. It's id will automatically be calculated.