Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  b : B
  bs : list B
  hd : A
  tl : list A
  tl' : list A
  h : OnOne2All P bs tl tl'
  i' : B
  x : A
  y : A
  u : list A
  v : list A
  a : P i' x y
  H : tl = u ++ x :: v
  H0 : tl' = u ++ y :: v
  ============================
   (hd :: tl = (hd :: u) ++ x :: v)


DEBUG:Going to execute:
<Coq.Init.Tauto.flatten_contravariant_conj> flags X1 X2 id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := B
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.