input: Write a function to calculate the greatest common divisor (GCD) of two numbers and return the result. If no numbers are provided (i.e., both numbers are zero), return 0.
  assert gcd(10, 20)==10
  assert gcd(15, 30)==15
  assert gcd(7, 13)==1
  assert gcd(0, 0)==0
  assert gcd(0, 5)==5
output: def gcd(a, b):
      if a == 0 and b == 0:
          return 0
      while b != 0:
          a, b = b, a % b
      return a