50 UpdateDifficultyTier calls
  frame=   2 cdiff_in= -1 store                            -1 -> 0  <== STORE
  frame=  49 cdiff_in=  0 no_store_unchanged               0 -> 0
  frame=  50 cdiff_in=  0 store                            0 -> 1  <== STORE
  frame=  51 cdiff_in=  1 no_store_unchanged               1 -> 1
frames seen: 2 .. 51