# Factorial without accumulator
def fac(n):
    if n == 0: return 1; else: return n * factorial(n - 1);

# Now with accumulator.
def t_fac(n):
    return t_fac_a(n, 1);
def t_fac_a(n, accum):
    if n == 0: return accum; else: return t_fac_a(n-1, n*accum);