function proposalExpirationTime(address wallet, MonetaryTypesLib.Currency memory currency)
    public
    view
    returns (uint256)
    {
        uint256 index = proposalIndexByWalletCurrency[wallet][currency.ct][currency.id];
        require(0 != index, "No proposal found for wallet and currency [NullSettlementChallengeState.sol:340]");
        return proposals[index - 1].expirationTime;
    }