input: from typing import List
  
  def binary_xor(a: str, b: str) -> str:
      """ Given two binary strings, a and b, return the result of their XOR operation as a string.
      >>> binary_xor('010', '110')
      '100'
      """
output:     return ''.join('1' if x != y else '0' for x, y in zip(a, b))