Goal:
  
  A : Type
  P : A → A → Prop
  a : A
  l : list A
  a0 : A
  l' : list A
  r : A
  r' : A
  IHl : Forall2 P l l' ∧ P r r'
  H : Forall2 P (a :: l ++ [r]) (a0 :: l' ++ [r'])
  H3 : P a a0
  H5 : Forall2 P (l ++ [r]) (l' ++ [r'])
  ============================
   (Forall2 P l l' → P r r' → Forall2 P (a :: l) (a0 :: l') ∧ P r r')


DEBUG:Going to execute:
intro