Goal:
    x : V.t  y : V.t  p : VSet.t  a : V.t
  ============================
   ((x = a \/ y = a \/ VSet.In a p -> y = a \/ x = a \/ VSet.In a p) /\
    (y = a \/ x = a \/ VSet.In a p -> x = a \/ y = a \/ VSet.In a p))


DEBUG:Going to execute:
match goal with
           | H:?x = ?x |- _ => clear H
           | H:?x = ?y |- _ => rewrite -> H in *; clear H
           end