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


DEBUG:Going to execute:
fun p =>
             DepElim.introduce p;
              DepElim.elim_case p || destruct p || (case p; clear p)