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


Number extend [
  my_factorial [
    (self < 2) 
        ifTrue: [ ^1 ]
        ifFalse: [                 
	    ^ (2 to: self) fold: [ :a :b | a * b ]
        ]
  ]
].
 
7 factorial printNl.
7 my_factorial printNl.