Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2 (λ (x : A) (y : B), R (f x) (g y)) [] [])


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in All2 (λ (x : A) (y : B), R (f x) (g y)) [] [])