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
    let H0 := CoreTactics.block in lt (hd :: tl) z)


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')