Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2 P tl tl'
  hd0 : A
  hd' : A
  tl0 : list A
  p : P hd0 hd'
  H : False
  ============================
   ({|
      pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
      pr2 := OnOne2_tl P hd tl tl' a
    |} =
    {|
      pr1 := {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |};
      pr2 := OnOne2_hd P hd0 hd' tl0 p
    |})


DEBUG:Going to execute:
hnf in id
Level 5: evaluation returns
hnf in id; CoreTactics.destruct_tele_eq id || destruct id
where
id := H
of type
constr
b := b
of type
constr
a := a
of type
constr
R := eq
of type
constr of type tacvalue