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