Goal:
  
  A : Type
  P : A → A → Prop
  r : A
  r' : A
  H : Forall2 P [r] [r']
  H3 : P r r'
  H5 : Forall2 P [] []
  ============================
   (Forall2 P [] [])


DEBUG:Going to execute:
assert X2; [ exact (id0 id1) | clear id0 ]