Goal:
  
  A : Type
  B : Type
  P : nat → A → B → Type
  l0' : list A
  l1' : list B
  H : All2i_len P l0' l1'
  x : A
  y : B
  l : list A
  l' : list B
  r : P (#|l| + #|l0'|) x y
  X : All2i_len (λ n : nat, P (n + #|l0'|)) l l'
  IHX : All2i_len P (l ++ l0') (l' ++ l1')
  ============================
   (P (#|l| + #|l0'|) x y)


DEBUG:Going to execute:
let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"