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


DEBUG:Going to execute:
push not in * using MSet_decidability