Goal:
  
  z : t
  hd : A.t
  tl : t
  tl' : t
  H : lt_ tl tl'
  IHlt_ : forall z : t, lt tl' z -> lt tl z
  H' : lt (hd :: tl') z
  ============================
   (lt (hd :: tl) z)


DEBUG:Going to execute:
match goal with
           | H':_ |- _ => match H' with
                          | H => idtac
                          end
           end