Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  a : All2i R (S n) l r
  r2 : R n x y
  b : All2i R (S n) l r
  ============================
   (NoConfusionHom_All2i A B R n (x :: l) (y :: r) 
      (All2i_cons r0 a) (All2i_cons r2 b)
    → let H := CoreTactics.block in All2i_cons r0 a = All2i_cons r2 b)


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end