Source : Free On-Line Dictionary of Computing

Peano arithmetic
     
         A system for representing {natural number}s
        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)