ser defined flag is represented as a String.
 User flags are case-independent. <p>

 A set of standard system flags are predefined.  Most folder
 implementations are expected to support these flags.  Some
 implementations may also support arbitrary user-defined flags.  The
 <code>getPermanentFlags</code> method on a Folder returns a Flags
 object that holds all the flags that are supported by that folder
 implementation. <p>

 A Flags object is serializable so that (for example) the
 use of Flags objects in