A layer is an irmin-pack store that can be opened as a normal store, or that can be combine with other layers, to form a layered store. As in Overlayfs, the lower layer is readonly, whereas the upper layer is used for adding the latest values. add is always performed on upper. find first tries to retrieve data from upper, and then defaults to lower.
Unlike Overlayfs, data from lower is never copied back in upper: every time a data from lower is needed it is accessed on lower.