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
  ============================
   (let H := CoreTactics.block in lt (hd :: tl) z)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end