Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type  b : All3 R [] [] []
  ============================
   (NoConfusionHom_All3 A B C R [] [] [] All3_nil b
    → let H := CoreTactics.block in All3_nil = b)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end