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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 btc_tx_der_encode_length(l):
 """ Return a DER-encoded length field Based on code from python-ecdsa (https://github.com/warner/python-ecdsa) by Brian Warner. Subject to the MIT license. """

    if l < 0:
        raise ValueError("length cannot be negative")

    if l < 0x80:
        return int2byte(l)
    s = ("%x" % l).encode()
    if len(s) % 2:
        s = b("0") + s
    s = binascii.unhexlify(s)
    llen = len(s)
    return int2byte(0x80 | llen) + s