Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → Type
  Q : B → C → Type
  R : A → C → Type
  l'' : list C
  a1' : list B
  a1 : list C
  a : All2 Q a1' a1
  ============================
   ({| pr1 := []; pr2 := l'' |} = {| pr1 := a1'; pr2 := a1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      (∀ (x : A) (y : B) (z : C), P x y → Q y z → R x z) → All2 R [] l'')


DEBUG:Going to execute:
fun tac p =>
             let ty := type of p in
             let eliminator := eval simpl in DepElim.elim in
             tac p eliminator