G-single-item #0
Let:
  T1 = {:index 20, :time 1690118453, :type :ok, :process 9, :f :txn, :value [[:r 9 nil] [:append 9 8]]}
  T2 = {:index 12, :time 1556729352, :type :ok, :process 7, :f :txn, :value [[:append 8 4] [:append 9 6]]}

Then:
  - T1 < T2, because T1 observed the initial (nil) state of 9, which T2 created by appending 6.
  - However, T2 < T1, because T1 appended 8 after T2 appended 6 to 9: a contradiction!