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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def hex2bin(hexstr):
 """Convert a hexdecimal string to binary string, with zero fillings. """

    num_of_bits = len(hexstr) * 4
    binstr = bin(int(hexstr, 16))[2:].zfill(int(num_of_bits))
    return binstr