Access to node entries is valid only after the node was activated.
And it fails otherwise.

--- FAIL: TestΔBTail (0.63s)
    --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d (0.00s)
        --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R (0.00s)
            --- FAIL: TestΔBTail/rebuild/T/T1/B0:a-B1:b→T/T1/T-T/B0:c-B1:d/_T{};R/_→T/T/B0:a (0.00s)
panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children [recovered]
        panic: BUG: T000000000000003a points to T000000000000000c as parent in trackSet, but not found in T000000000000000c children