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:
destruct id as [idx id]; repeat CoreTactics.destruct_sigma idx;
            simpl in id