toFormula (Iff a b)                = join $ Z.mkIff <$> toFormula a <*> toFormula b
toFormula (Iff a b)                = join $ Z.mkIff <$> toFormula a <*> toFormula b
toFormula (Iff a b)                = join $ Z.mkIff <$> toFormula a <*> toFormula b