input: Write a function to calculate the greatest common divisor (GCD) of two numbers and return the result. If one of the numbers is zero, return the other number as the GCD. 
  
  def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b)
  
  assert gcd(10, 20)==10
  assert gcd(15, 25)==5
  assert gcd(0, 7)==7
  assert gcd(8, 0)==8
output: def gcd(a, b):
      if b == 0: 
          return a 
      return gcd(b, a % b)