What if roots achieve ROOT_HISTORY_SIZE and rewrite roots[0]? #4

 function _insert(bytes32 _leaf) internal returns (uint32 index) {
        uint32 currentIndex = nextIndex;
        require(currentIndex < uint32(2)**levels, "Merkle tree is full. No more leaf can be added");
        nextIndex += 1;
        bytes32 currentLevelHash = _leaf;
        bytes32 left;
        bytes32 right;

        for (uint32 i = 0; i < levels; i++) {
            if (currentIndex % 2 == 0) {
                left = currentLevelHash;
                right = zeros[i];
                filledSubtrees[i] = currentLevelHash;
            } else {
                left = filledSubtrees[i];
                right = currentLevelHash;
            }

            currentLevelHash = hashLeftRight(left, right);
            currentIndex /= 2;
        }

        currentRootIndex = (currentRootIndex + 1) % ROOT_HISTORY_SIZE; // this line
        roots[currentRootIndex] = currentLevelHash;
        return nextIndex - 1;
    }