X ΔFtail: `go test -failfast -short -v -run Random -randseed=1626793016249041295` discovered problems

=== RUN   TestΔFtailRandom
    δftail_test.go:141: # n=10 seed=1626793016249041295
2021/07/23 12:26:01 zodb: FIXME: open /tmp/TestΔFtailRandom1363232041/001/1.fs: raw cache is not ready for invalidations -> NoCache forced
    δftail_test.go:191: #   @at0 (03e19cb6064c58dd)
    δftail_test.go:203: # → @at1 (03e19cb6064ddd99)  t0:a Da:a,b:b,c:c,d:d,e:e,f:f,g:g,h:h,i:i,j:j      ; not-yet-tracked
    δftail_test.go:375: # → @at2 (03e19cb6064fc922)  δT2:i,3:c,5:d,9:c  δD{a b c d e f g h i}   ; t0:a,2:i,3:c,5:d,9:c Da:a2,b:b2,c:c2,d:d2,e:e2,f:f2,g:g2,h:h2,i:i2,j:j        δ{0 2 3 5 9}
    δftail_test.go:472: δf:
        have: &{03e19cb6064fc922 false {2 3 5 9} true}
        want: &{03e19cb6064fc922 false {0 2 3 5 9} true}
    δftail_test.go:499: .trackSetZBlk:
        ~have: map[c:{3 9} d:{5} i:{2}]
         want: map[a:{0} c:{3 9} d:{5} i:{2}]

    ...