-- This program takes a number from standard input and returns
-- the factorial of this number to standard output.
-- Note that 0 is returned for a negative input.

INT factorial:
INT result=0:
SEQ
  stdin ? factorial -- input factoral value to be calculated
  IF
    factorial >= 0
      result := 1
  WHILE factorial > 0
    SEQ
      result := (result * factorial)
      factorial := (factorial-1)
  stdout ! result