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