Goal:
  
  x : V.t
  p : VSet.t
  H : VSet.In x p -> False
  a : V.t
  H0 : x = a -> False
  ============================
   (VSet.In a p -> (x = a \/ VSet.In a p) /\ x <> a)


DEBUG:Going to execute:
<coq-core.plugins.ltac::intro@0>