code: def extendedEuclid(a, b):
      
      if a == 0:
          return b, 0, 1
      else:
          g, y, x = extendedEuclid(b % a, a)
          return g, x - (b // a) * y, y
docs: return a tuple of three values: x, y and z, such that x is
      the GCD of a and b, and x = y * a + z * b