Natural numbers

natural.sml

abstype natural = next of natural | zero of unit
	with
	fun   sum (zero(), n)  = n
	    | sum (next(n), m) = next(sum(m ,n));

	fun   prod(zero(), n) = zero()
	    | prod(next(n),m) = sum(prod(n,m), m);

	fun   fact(zero())       = next(zero())
	    | fact(next(zero())) = fact(zero())
	    | fact(next(n))      = prod(next(n), fact(n));

	val fact_3 = fact(next(next(next(zero()))));

end