Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  l' : list B
  r : R #|l| x y
  X : All2i_len R l l'
  IHX : All2i R 0 (List.rev l) (List.rev l')
  ============================
   (All2i R (0 + #|List.rev l|) [x] [y])


DEBUG:Going to execute:
solve
[ do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]
| case not_locked_false_eq_true; Ltac.assumption
| match goal with
  | H:¬ _ |- _ => solve [ case H; trivial ]
  end ]