Goal:
  
  A : Type
  R : A → A → Type
  a : A
  l : list A
  IHl : All2 R l l → All (λ x : A, R x x) l
  ============================
   (All2 R (a :: l) (a :: l) → All (λ x : A, R x x) (a :: l))


DEBUG:Going to execute:
inversion 1; constructor