(Program
{+(Return
  {+(
    {+(Plus
      {+(Identifier)+}
      {+(Identifier)+})+}
    {+(Identifier)+})+})+}
  (Return
    (Empty))
  (Return
  { (
    {-(Plus
      {-(Identifier)-}
      {-(Identifier)-})-}
    {-(Identifier)-})
  ->(Not
    {+(Identifier)+}) })
{-(Return
  {-(Not
    {-(Identifier)-})-})-})