SubRegion roi assumes Array-like slot type

[bekaus]

Inside the init of SubRegion it is assumed, that the slot is ArrayLike. This is in general a wrong assumption since non array-like slot can also have SubRegion rois. Breaks the slot typing system.

[cstraehl]

i would argue that types of regions of interests should always be coupled to slot types.

in any case, this should be done more explicitly.