Goal:
  
  G : t
  idx0 : VSet.t
  idx : sigma (fun _ : V.t => V.t)
  a : SPath (pr1 {| pr1 := idx0; pr2 := idx |})
        (pr1 (pr2 {| pr1 := idx0; pr2 := idx |}))
        (pr2 (pr2 {| pr1 := idx0; pr2 := idx |}))
  b : sigma
        (fun index : sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t)) =>
         SPath (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  H : NoConfusion_SPath {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |}
        b
  ============================
   ({| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |} = b)


DEBUG:Going to execute:
fun id =>
             match type of id with
             | sigma ?P =>
                 let idx := fresh "idx" in
                 destruct id as [idx id];
                  repeat CoreTactics.destruct_sigma idx; 
                  simpl in id
             | _ => idtac
             end