Goal:
  
  G : t
  s0 : VSet.t
  x : V.t
  s1 : VSet.t
  x0 : V.t
  H : {| pr1 := s0; pr2 := x |} = {| pr1 := s1; pr2 := x0 |}
  ============================
   ({|
      pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
      pr2 := spath_refl s0 x
    |} =
    {|
      pr1 := {| pr1 := s1; pr2 := {| pr1 := x0; pr2 := x0 |} |};
      pr2 := spath_refl s1 x0
    |})


DEBUG:Going to execute:
match type of H with
           | ?R ?x ?y =>
               let rhs := fresh in
               set (rhs := y) in *; pattern sigma rhs; clearbody rhs;
                destruct H; simpl
           end