just understand that no overwrite there, it create more than one tree, each node on xyzw which is real confusing
but still, if there is anyone could tell just how broad subkernel could be used
there is no such novel use in docs, like "a stream can be written inside a parent kernel by subkernel to be read again by parent kernel. As by prior kernel operation, we can conclude it can be writeback to be readback as long as it is within a kernel/subkernel, still with one-to-one mapping"
edit: in subkernel there is no thread mapping, it just enables us to write in parent stream position
again, it is hard