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