Goal:
  
  A : Type
  P : A → A → Type
  l' : list A
  f : A → A
  hp : ∀ x y : A, P (f x) y → ∑ y' : A, y = f y'
  o1 : sigma (λ _ : list A, list A)
  o : OnOne2 P (pr1 o1) (pr2 o1)
  ============================
   ({| pr1 := []; pr2 := l' |} = o1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) [] 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 := o1
of type
constr
id' := o
of type
ident
id := o
of type
ident
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := o
        of type
        ident
of type
tacvalue of type tacvalue