====== Natural numbers ======
{{tag>math numbers recursion}}
===== Description =====
Defining the natural numbers.

**Source**:  The Art of Prolog
===== Download =====
Program source code: {{natural_numbers.pl}}
===== Listing =====
<code prolog>
/* 
	natural_number(X) :- X is a natural number.
*/

	natural_number(0).
	natural_number(s(X)) :- natural_number(X).

%	Program 3.1:  Defining the natural numbers
</code>
===== Comments =====