A re-drill replaces one region's refs
One RefMap belongs to snapshot s2txcl95uvubmn. It holds the skeleton refs e1 and e2, the refs e3 to e33 from a drill of e1, and refs from drills of other roots. Running snapshot --root on e1 again calls remove_by_root_ref, which removes e3 to e33, then allocates e34 to e64 from the same counter. The skeleton refs, including e1 itself, and the refs of other regions stay valid. The old e3 to e33 now return STALE_REF.
snapshot --root @s2txcl95uvubmn:e1
re-drill after an action
RefMap for snapshot s2txcl95uvubmn
skeleton
no root_ref
@e1, @e2
kept, e1 included
drill of @e1
root_ref: @e1
@e3 … @e33
removed: STALE_REF
remove_by_root_ref
@e34 … @e64
new ids, counter continues
other regions
root_ref: another ref
their refs
kept, still valid