Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  hd' : A.t
  tl' : list A.t
  H : A.lt hd hd'
  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))
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in lt (hd :: tl) 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)