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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 bitstring_to_bytes(bitstr):
 """ Converts a pyasn1 univ.BitString instance to byte sequence of type 'bytes'. The bit string is interpreted big-endian and is left-padded with 0 bits to form a multiple of 8. """

    bitlist = list(bitstr)
    bits_missing = (8 - len(bitlist) % 8) % 8
    bitlist = [0]*bits_missing + bitlist # pad with 0 bits to a multiple of 8
    result = bytearray()
    for i in range(0, len(bitlist), 8):
        byte = 0
        for j in range(8):
            byte = (byte << 1) | bitlist[i+j]
        result.append(byte)
    return bytes(result)