code: def btc_tx_der_encode_length(l):
      
      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
docs: 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.