Definitions

   The factorial of   0   (zero)   is defined as being   1   (unity).
   The   Factorial Function   of a positive integer,    n,    is defined as the product of the sequence:
                 n,   n-1,   n-2,   ...   1 



Task

Write a function to return the factorial of a number.

Solutions can be iterative or recursive.

Support for trapping negative    n    errors is optional.



Related task

   Primorial numbers


 
fact
 if n < 1
    return
 endif
  * n fn fn
 - n
 call fact
 return
 
start
 
 if i < 20
    1 fn
    i n
    call fact
    fn []
    + i
    goif
 endif
 