public sealed class NumberSystem
    // The permits clause has been omitted
    // as all the subclasses exists in the same file.
{ }
final class Binary extends NumberSystem { .. }
final class Octal extends NumberSystem { .. }
final class HexaDecimal extends NumberSystem { .. }
sealed class Decimal extends NumberSystem { .. }
final class NonRecurringDecimal extends Decimal {..}
final class RecurringDecimal extends Decimal {..}