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))
  H : NoConfusion_SPath
        {|
          pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
          pr2 := a
        |} {| pr1 := idx2; pr2 := b |}
  ============================
   ({|
      pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
      pr2 := a
    |} = {| pr1 := idx2; pr2 := b |})


DEBUG:Going to execute:
let idx := fresh "idx" in
           destruct id as [idx id]; repeat CoreTactics.destruct_sigma idx;
            simpl in id
DEBUG LTAC Evaluated term: idx2
DEBUG LTAC Evaluated term: (sigma
                              (fun _ : VSet.t => sigma (fun _ : V.t => V.t)))
Level 6: type of id
 has value 
(sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t)))