code: public function setFrameFocusByIndex($choice)
      {
          if (is_integer($this->focus)) {
              if ($this->frames[$this->focus]->hasFrames()) {
                  return $this->frames[$this->focus]->setFrameFocusByIndex($choice);
              }
          }
          if (($choice < 1) || ($choice > count($this->frames))) {
              return false;
          }
          $this->focus = $choice - 1;
  
          return true;
      }
docs: Sets the focus by index. The integer index starts from 1.
  If already focused and the target frame also has frames,
  then the nested frame will be focused.
  
  @param int $choice    Chosen frame.
  
  @return bool           True if frame exists.