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


DEBUG:Going to execute:
try unfold signature in id'2