Mine

factorial[... end n]
	factorial_internal(... end 0 n)

{all of this should be easier with anonymous functions}
factorial_internal[... n]
	if(... n stop cont n)
cont[... end res n]
	add(... end n factorial_internal_second res n)
stop[... end res n]
	end(... res)
factorial_internal_second[... end n res]
	subtract(... end res factorial_internal n 1)