Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  H1' : t
  H1 : t
  H : lt_ (pr1 {| pr1 := H1'; pr2 := H1 |}) (pr2 {| pr1 := H1'; pr2 := H1 |})
  ============================
   ({| pr1 := hd :: tl; pr2 := z |} = {| pr1 := H1'; pr2 := H1 |} ->
    let H := CoreTactics.block in let H0 := CoreTactics.block in lt [] z)


DEBUG:Going to execute:
DepElim.decompose_exists xn id