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


begin
	comment factorial - algol 60;
	integer procedure factorial(n); integer n;
	begin
		integer i,fact;
		fact:=1;
		for i:=2 step 1 until n do
			fact:=fact*i;
		factorial:=fact
	end;
	integer i;
	for i:=1 step 1 until 10 do outinteger(1,factorial(i));
	outstring(1,"\n")
end