def add_txin_info(cls, swap, txin: PartialTxInput) -> None:
        """Add some info to a claim txin.
        note: even without signing, this is useful for tx size estimation.
        """
        preimage = swap.preimage if swap.is_reverse else 0
        witness_script = swap.redeem_script
        txin.script_sig = b''
        txin.witness_script = witness_script
        sig_dummy = b'\x00' * 71  # DER-encoded ECDSA sig, with low S and low R
        witness = [sig_dummy, preimage, witness_script]
        txin.witness_sizehint = len(construct_witness(witness))
        txin.nsequence = 1 if swap.is_reverse else 0xffffffff - 2
Add some info to a claim txin.
        note: even without signing, this is useful for tx size estimation.
        
electrum/submarine_swaps.py