FACT()
 Computes the factorial
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 Syntax

     FACT(<nValue>) --> nFactorial

 Argument

     <nValue>  Designates a value between 0 and 21.

 Returns

     FACT() returns the factorial of the specified value.

 Description

     FACT() computes the factorial of a value in the range of 0 to 21.  Since
     results for factorials over 21 are too large, the function returns -1 to
     indicate an error.

 Examples

     Show some factorials:

     ? FACT(1)         // Result: 1
     ? FACT(5)         // Result: 120
     ? FACT(21)        // Result: 51090942171709440000
     ? FACT(25)        // Result: -1 (Value too large !)
     ? FACT(0)         // Function returns 1