Fact - script and chart function
                Fact() returns the factorial of a positive integer x.
Syntax:
Fact(x)
Return data type: integer
Limitations:
If the number x is not an integer, it will be truncated. Non-positive numbers will return NULL.
Examples and results:
| Examples | Results | 
|---|---|
| 
                                 Fact( 1 )  | 
                            
                                 Returns 1  | 
                        
| 
                                 Fact( 5 )  | 
                            Returns 120 ( 1 * 2 * 3 * 4 * 5 = 120 ) | 
| 
                                 Fact( -5 )  | 
                            
                                 Returns NULL  |