Goal:
  
  G : t
  s0 : VSet.t
  x : V.t
  idx3 : VSet.t
  idx4 : V.t
  idx2 : V.t
  b : SPath idx3 idx4 idx2
  H : NoConfusion_SPath
        {|
          pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
          pr2 := spath_refl s0 x
        |}
        {|
          pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
          pr2 := b
        |}
  ============================
   (let H := CoreTactics.block in
    {|
      pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
      pr2 := spath_refl s0 x
    |} =
    {|
      pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
      pr2 := b
    |})


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end