Goal:
  
  G : t
  idx0 : VSet.t
  idx1 : V.t
  idx : V.t
  a : SPath idx0 idx1 idx
  idx2 : sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t))
  b : SPath (pr1 idx2) (pr1 (pr2 idx2)) (pr2 (pr2 idx2))
  e : NoConfusion_SPath
        {|
          pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
          pr2 := a
        |} {| pr1 := idx2; pr2 := b |}
  ============================
   (noConfusion_SPath_obligation_2
      {|
        pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
        pr2 := a
      |} {| pr1 := idx2; pr2 := b |}
      (noConfusion_SPath_obligation_1
         {|
           pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
           pr2 := a
         |} {| pr1 := idx2; pr2 := b |} e) = e)


DEBUG:Going to execute:
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