Currently fails with e.g.:

(neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree$ go test -short -v -failfast -run 'TestΔBTail$/Update/T3/B1:a-B8:c→T7/B1:a-B8:c'
=== RUN   TestΔBTail
2021/06/23 14:08:45 zodb: FIXME: open /tmp/δBTail926507346/1.fs: raw cache is not ready for invalidations -> NoCache forced
=== RUN   TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c
    δbtail_test.go:812: BUG: tkeyCov1 ⊄ tkeyCov2:
                tkeyCov1: {[3,∞)}
                tkeyCov2: {[7,∞)}
                tkeyCov1 \ tkeyCov2: {[3,7)}
    δbtail_test.go:708: T3/B1:a-B8:c -> T7/B1:a-B8:c  ; tracked={8} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 8 9223372036854775807} 8:{1 8 9223372036854775807} 9223372036854775807:{1 8 9223372036854775807}]

--- FAIL: TestΔBTail (0.84s)
    --- FAIL: TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c (0.00s)