/**
  * Handles the key down of a keyboard modifier
  *
  * @param string $modifier The modifier to handle (see self::MODIFIER_*)
  */
 protected function keyDownModifier($modifier)
 {
     switch ($modifier) {
         case self::MODIFIER_CTRL:
             throw new UnsupportedDriverActionException('Ctrl key is not supported by %s', $this);
         case self::MODIFIER_ALT:
             $this->browser->altKeyDown();
             break;
         case self::MODIFIER_SHIFT:
             $this->browser->shiftKeyDown();
             break;
         case self::MODIFIER_META:
             $this->browser->metaKeyDown();
             break;
     }
 }