Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  HPQ : ∀ x y : A, Q x → P x y → Q y
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  H1 : list A
  H : All Q H1
  ============================
   (hd :: tl = H1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All Q (hd' :: tl))


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 := hd :: tl
of type
constr
id'1 := H1
of type
constr
id' := H
of type
ident
id := H
of type
ident
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := H
        of type
        ident
of type
tacvalue of type tacvalue