tree_lens.cpp

struct VolumeVolumeRegistration;
struct VolumeTree {
    Volume volume;
    std::vector<VolumeVolumeRegistration> children;
    size_t depth = 0;
};

struct VolumeVolumeRegistration {
    Transform transform;
    VolumeTree tree;
};

// [...]

    auto volumeLensAt(VolumeTree tree, int &index)
        -> std::optional<std::decay_t<decltype(lager::lens::attr(&VolumeTree::volume))>>
    {
        using namespace lager::lens;
        if (index == 0) {
            return attr(&VolumeTree::volume);
        } else {
            size_t i = 0;
            for (auto child : tree.children) {
                --index;
                auto resLens = volumeLensAt(child.tree, index);
                if (resLens.has_value()) {
                    return attr(&VolumeTree::children)
                        | at(i)
                        | attr(&VolumeVolumeRegistration::tree)
                        | resLens.value();
                }
                ++i;
            }
            return std::nullopt;
        }
    }