Function            factorial - determines a factorial (n!)

  Syntax              double factorial(int n);

  Prototype in        mathhk.h

  Remarks             factorial returns n! (1*2*3*...*n). Traditionally
                      seen as a recursive function used to illustrate
                      recursion, factorial is done with a loop, using no
                      recursion.

  Return value        returns n factorial (n! = 1*2*3*...*n). If n <= 1,
                      factorial returns 1. If the result of n! is larger
                      than MAXDOUBLE (the largest value a double can
                      store, defined in Borland's LIMITS.H), factorial
                      return zero.

  See also            summation()

  Example             #include <mathhk.h>

                      main()
                      {
                           double f;

                           if ((f = factorial(13)) == 0)
                                printf("Error computing 13!\n");
                           else
                                printf("13! = %15.0lf\n",factorial(13));
                      }

  Program output      13! = 6227020800