Goal:
  
  A : Type
  R : A → A → Prop
  a : A
  l : list A
  H : Forall (λ x : A, R x x) (a :: l)
  x : A
  l0 : list A
  H2 : R a a
  H3 : Forall (λ x : A, R x x) l
  H0 : x = a
  H1 : l0 = l
  H4 : Forall2 R l l
  ============================
   (Forall2 R l l)


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 0: Backtrace:

R is used in conclusion.
Level 0: Backtrace:

R is used in conclusion.
Level 0: Backtrace:

R is used in conclusion.
Level 0: Backtrace:

R is used in conclusion.
Level 0: Backtrace:

R is used in conclusion.
Level 1: Backtrace:

R is used in conclusion.
Level 0: Backtrace:

R is used in conclusion.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 1: Backtrace:

Not convertible.