uint256 largestTrancheIdx;
uint256 largestPrincipal;
for (uint256 i = 0; i < _loan.tranche.length;) {
    if (_loan.tranche[i].principalAmount > largestPrincipal) {
        largestPrincipal = _loan.tranche[i].principalAmount;
        largestTrancheIdx = i;
    }
    unchecked {
        ++i;
    }
}
tranche = [
  0: 20 eth from Bob
  1: 20 eth from Alice
  2: 0.01 eth from Caleb
  3: 20 eth from Alice
]
