code: def _btc_witness_serialize_unit(unit):
      
      
      if isinstance(unit, int):
          # pass literal
          return encoding.from_int_to_byte(unit)
  
      elif unit is None:
          # None means OP_0
          return b'\x00'
  
      else:
          # return as a varint-prefixed string
          return make_var_string(unit)
docs: Encode one item of a BTC witness script
      Return the encoded item (as a string)
  
      Returns a byte string with the encoded unit
  
      Based on code from pybitcointools (https://github.com/vbuterin/pybitcointools)
      by Vitalik Buterin