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


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