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