Goal:
  
  A : Type
  B : Type
  C : Type
  P : nat → A → B → Type
  Q : B → C → Type
  R : nat → A → C → Type
  S : nat → A → C → Type
  l'' : list C
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : P n x y
  a : All2i P (Datatypes.S n) l r
  b : All2 Q (y :: r) l''
  c : All2i R n (x :: l) l''
  H : ∀ (n : nat) (x : A) (y : B) (z : C),
        P n x y → Q y z → R n x z → S n x z
  IHa : ∀ l'' : list C,
          All2 Q r l''
          → All2i R (Datatypes.S n) l l'' → All2i S (Datatypes.S n) l l''
  ============================
   (All2i S n (x :: l) l'')


DEBUG:Going to execute:
intro_block_id H || intro_block H
Level 0: Backtrace:

No quantified hypothesis named b in current goal even after head-reduction.
Level 0: Backtrace:

No quantified hypothesis named b in current goal even after head-reduction.