prenex (Not p)       = let Prenex quants p' = prenex p
                       in Prenex (map flipQuantifier quants) (PNot p')
prenex (Not p)       = let Prenex quants p' = prenex p
                       in Prenex (map flipQuantifier quants) (PNot p')
prenex (Not p)       = let Prenex quants p' = prenex p
                       in Prenex (map flipQuantifier quants) (PNot p')