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'1' : t
  H'1 : t
  H' : lt_ (pr1 {| pr1 := H'1'; pr2 := H'1 |})
         (pr2 {| pr1 := H'1'; pr2 := H'1 |})
  ============================
   ({| pr1 := hd :: tl'; pr2 := z |} = {| pr1 := H'1'; pr2 := H'1 |} ->
    let H := CoreTactics.block in
    let H0 := CoreTactics.block in lt (hd :: tl) z)


DEBUG:Going to execute:
DepElim.decompose_exists xn id