input: Write a function that computes the greatest common divisor (GCD) of two numbers using the Euclidean algorithm. The function should take two integers as input and return the GCD of the two numbers. If either of the numbers is zero, the function should return 0 regardless of the other number. 
  
  assert gcd(0, 5) == 0
  assert gcd(10, 20) == 10
  assert gcd(7, 13) == 1
  assert gcd(0, 0) == 0
  assert gcd(36, 60) == 12
output: def gcd(a, b):
      if a == 0 or b == 0:
          return 0
      while b != 0:
          a, b = b, a % b
      return a