Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All3_nil = All3_nil)


DEBUG:Going to execute:
match T with
           | context [ CoreTactics.block ] =>
               change T; CoreTactics.intros_until_block
           end