private function getPrefixRules( $prop, $val, $extra = null )
  {
    $extra = ( empty( $extra ) ) ? null : ' ' . $extra;
    if( !is_array( $prop ) && is_array( $val ) )
    {
      $properties = array_fill( 0, count( $val ), $prop );
      $values     = $val;
    }
    elseif( !is_array( $val ) && is_array( $prop ) )
    {
      $values     = array_fill( 0, count( $prop ), $val );
      $properties = $prop;
    }
    else
    {
      $properties = $prop;
      $values     = $val;
    }

    $property = null;
    $value    = null;
    do
    {
      $property = ( !empty( $properties ) ) ? array_shift( $properties ) : $property;
      $value    = ( !empty( $values ) ) ? array_shift( $values ) : $property;

      $rules[] = new CssRule( $property, $value, $extra );
    }
    while( !empty( $properties ) && !empty( $values ) );

    return $rules;
  }