Goal:
  
  G : t
  idx0 : VSet.t
  idx1 : V.t
  idx : V.t
  a : SPath (pr1 {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |})
        (pr1 (pr2 {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |}))
        (pr2 (pr2 {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |}))
  ============================
   (NoConfusion_SPath
      {|
        pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
        pr2 := a
      |}
      {|
        pr1 := {| pr1 := idx0; pr2 := {| pr1 := idx1; pr2 := idx |} |};
        pr2 := a
      |})


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