Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  P : A → Prop
  x : A
  y : B
  l : list A
  l' : list B
  H : R x y
  H0 : Forall2 R l l'
  H2 : P x
  H3 : Forall P l
  H1 : Forall2 (λ (x : A) (y : B), P x ∧ R x y) l l'
  ============================
   (R x y)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
            [ intro; clear id
            | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
Level 2: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := A
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.