code: protected function addRadioButton($tag)
      {
          if (! isset($this->radios[$tag->getName()])) {
              $this->widgets[]               = new SimpleRadioGroup();
              $this->radios[$tag->getName()] = count($this->widgets) - 1;
          }
          $this->widgets[$this->radios[$tag->getName()]]->addWidget($tag);
      }
docs: Adds a radio button, building a group if necessary.
  
  @param SimpleRadioButtonTag $tag   Incoming form control.