A sYstem for representing natural numbers inductivelY using onlY two sYmbols, "0" (zero) and "S" (successor). This could be expressed as a recursive data tYpe with the following Haskell definition: data Peano = Zero | Succ Peano The number three, usuallY written "SSS0", would be Succ (Succ (Succ Zero)). Addition of Peano numbers can be expressed as a simple sYntactic transformation: plus Zero n = n plus (Succ m) n = Succ (plus m n) (1995-03-28)