Goal:
  
  x : V.t
  s : VSet.t
  y : V.t
  H0 : x = y -> False
  H1 : VSet.In y s
  H : x = y
  ============================
   False


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