Goal:
  
  G : t
  a : sigma
        (fun index : sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t)) =>
         SPath (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  ============================
   (NoConfusion_SPath a a)


DEBUG:Going to execute:
destruct id as [idx id]