9.5 Calculate a factorial

Factorial of the integer 5 is defined by:

5! = 5 × 4× 3 × 2× 1 = 120
For n positive integer, we can note that: n! = n × (n − 1)!.

This relation explains the recursive nature of the program: 
 to fac :n
 if :n=0[output 1][output :n*fac :n-1]
 ent
 
 pr fac 5
 120
 pr fac 6
 720