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


const func bigInteger: factorial (in bigInteger: n) is func
  result
    var bigInteger: fact is 1_;
  local
    var bigInteger: i is 0_;
  begin
    for i range 1_ to n do
      fact *:= i;
    end for;
  end func;