Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type  H : True
  ============================
   (let H := CoreTactics.block in All3_nil = All3_nil)


DEBUG:Going to execute:
CoreTactics.block_goal
Level 8: evaluation returns
idtac of type tacvalue

Level 6: evaluation returns
idtac of type tacvalue

Level 5: evaluation returns
idtac of type tacvalue