Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  H1 : sigma (fun _ : t => t)
  H : lt_ (pr1 H1) (pr2 H1)
  ============================
   ({| pr1 := hd :: tl; pr2 := z |} = H1 ->
    let H := CoreTactics.block in let H0 := CoreTactics.block in lt [] 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