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


DEBUG:Going to execute:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
            hnf in (value of id'1), (value of id'2);
            try red in (type of id'2);
            match goal with
            | id'1:=?t:_
              |- _ =>
                  generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear
                   id' id; try unfold signature in id'2; 
                   hnf in id'2; simpl in id'2; rename id'2 into id;
                   cont id id id'1 t
            end