Lists merge

   protected function merge(array $x, array $y)
   {
      $result = [];
      while(count($x) || count($y))
      {
         if(count($x) && count($y))
         {
            if(current($x)<=current($y))
            {
               $result[] = current($x);
               unset($x[key($x)]);
            }
            else
            {
               $result[] = current($y);
               unset($y[key($y)]);
            }
         }
         elseif(count($x))
         {
            $result[] = current($x);
            unset($x[key($x)]);
         }
         elseif(count($y))
         {
            $result[] = current($y);
            unset($y[key($y)]);
         }
      }
      return $result;
   }