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))
  H1 := {| pr1 := hd :: tl; pr2 := z |} : sigma (fun _ : t => t)
  H2 := H
     : (fun index : sigma (fun _ : t => t) => lt_ (pr1 index) (pr2 index)) H1
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in lt [] z)


DEBUG:Going to execute:
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