Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → B → C → Type
  b : All3 R [] [] []
  H : 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