Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → Type
  Q : B → C → Type
  R : A → C → Type
  l'' : list C
  a1 : sigma (λ _ : list B, list C)
  a : All2 Q (pr1 a1) (pr2 a1)
  ============================
   ({| pr1 := []; pr2 := l'' |} = 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 [] l'')


DEBUG:Going to execute:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := {| pr1 := []; pr2 := l'' |}
of type
constr
id'1 := a1
of type
constr
id' := a
of type
ident
id := a
of type
ident
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := a
        of type
        ident
of type
tacvalue of type tacvalue