Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  hd' : A.t
  tl' : list A.t
  H : A.lt hd hd'
  H'1' : list A.t
  H'1 : list A.t
  H' : lt_ H'1' 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:
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