Goal:
  
  x : V.t
  s : VSet.t
  y : V.t
  H0 : x = y
  H : VSet.In y s
  H2 : x = y -> False
  ============================
   (x = y \/ VSet.In y s)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end