Goal:
  
  A : Type
  R : A → A → Prop
  a : A
  l : list A
  IHl : Forall (λ x : A, R x x) l → Forall2 R l l
  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
  ============================
   (R a a)


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end