Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  hd' : A.t
  tl' : list A.t
  H : A.lt hd hd'
  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