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Given the following text description, write Python code to implement the functionality described.
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Description:
 Input are two strings a and b consisting only of 1s and 0s.
    Perform binary XOR on these inputs and return result also as a string.
    
This is how the function will work:
 string_xor('010', '110')
    '100'
    
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Python Code:
from typing import List
def string_xor(a: str, b: str) -> str:
    
    def xor(i, j):
        if i == j:
            return '0'
        else:
            return '1'
    return ''.join(xor(x, y) for x, y in zip(a, b))
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