Goal:
  
  z : t
  hd : A.t
  tl : t
  tl' : t
  H : lt_ tl tl'
  IHlt_ : forall z : t, lt tl' z -> lt tl z
  H' : lt (hd :: tl') z
  H'0 := {| pr1 := {| pr1 := hd :: tl'; pr2 := z |}; pr2 := H' |}
      : sigma
          (fun index : sigma (fun _ : t => t) => lt_ (pr1 index) (pr2 index))
  H'1 := {| pr1 := hd :: tl'; pr2 := z |} : sigma (fun _ : t => t)
  H'2 := H'
      : (fun index : sigma (fun _ : t => t) => lt_ (pr1 index) (pr2 index))
          H'1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in lt (hd :: tl) 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