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


DEBUG:Going to execute:
inversion 1; constructor