Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → Type
  Q : B → C → Type
  R : A → C → Type
  l'' : list C
  x : A
  y : B
  l : list A
  l' : list B
  r : P x y
  X : All2 P l l'
  IHX : ∀ l'' : list C,
          All2 Q l' l''
          → (∀ (x : A) (y : B) (z : C), P x y → Q y z → R x z) → All2 R l l''
  a : All2 Q (y :: l') l''
  a0 := {| pr1 := {| pr1 := y :: l'; pr2 := l'' |}; pr2 := a |}
     : sigma
         (λ index : sigma (λ _ : list B, list C),
            All2 Q (pr1 index) (pr2 index))
  a1 := {| pr1 := y :: l'; pr2 := l'' |} : sigma (λ _ : list B, list C)
  a2 := a
     : (λ index : sigma (λ _ : list B, list C),
          All2 Q (pr1 index) (pr2 index)) a1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    (∀ (x : A) (y : B) (z : C), P x y → Q y z → R x z) → All2 R (x :: l) l'')


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2; hnf in id'2; 
            simpl in id'2; rename id'2 into id; cont id id id'1 t
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.
Level 2: Backtrace:

a is used in conclusion.