input: Write a function to compute the greatest common divisor (GCD) of two numbers using recursion.
  assert gcd(10, 15) == 5
  assert gcd(21, 14) == 7
  assert gcd(100, 25) == 25
output: def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)