docs: def tx_sign_input(tx_hex, idx, prevout_script, prevout_amount, private_key_info, blockchain='bitcoin', **blockchain_opts):
      """
      Sign a given input in a transaction, given the previous output script and previous output amount.
      Different blockchains can require additional fields; pass thse in **blockchain_opts.
  
      Return the serialized tx with the given input signed on success
      Raise on error
      """
      if blockchain == 'bitcoin':
          return btc_tx_sign_input(tx_hex, idx, prevout_script, prevout_amount, private_key_info, **blockchain_opts)
      else:
          raise ValueError('Unknown blockchain "{}"'.format(blockchain))