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


DEBUG:Going to execute:
hnf in id;
            match type of id with
            | sigma _ =>
                let xn := fresh id "'" in
                destruct id as [xn id]; DepElim.decompose_exists xn id;
                 cbv beta iota delta [TODO projection TODO projection]
                  in id, id'; DepElim.decompose_exists id id'
            | _ =>
                cbv beta iota delta [TODO projection TODO projection]
                 in id, id'
            end