* indicates abstract classes (in namespace Generics)
+ on leaves: indicates non-trivial subclassing
<blank line> indicates progress ;-)

r indicates rfc2822/822 standard headers
m indicates rfc2045/et al. proposed standard headers
u indicates usefor headers

+ Base*                 // defines common interface
  + GUnstructured*      // Base Class for unstructured header fields
    + Generic+          // can hold any header field
r   + Subject
r   + Comment
m   + ContentDescription
    + Organization     // must also be registered as "Organisation"
    + UserAgent
  + GStructured*       // Base Class for structured header fields
    + GAddress*        // Base Class for address-type header fields
      + MailboxList*   // doesn't allow groups: From
r       + From
	+ SingleMailbox* // only allows a single mailbox: Sender
r         + Sender
      + AddressList*   // allows groups: ReplyTo, To, Cc, Bcc
r       + ReplyTo
r       + To
r       + Cc
r       + Bcc
	+ MailCopiesTo // ??? listed in usefor ???
r     + ReturnPath+    // allows only a single angle-addr
      + GIdent*        // Base Class for "Identification Fields"
	+ GSingleIdent* // allows only a single msg-id
r         + MessageID
m         + ContentID
u?        + Supersedes
r       + InReplyTo
r       + References
    + GToken*          // Base Class for single-token headers: CTE
m     + ContentTransferEncoding
    + GDotAtom*        // Base Class for single-dot-atom headers: MIME-Version
m     + MIMEVersion
    + GPhraseList*     // Base Class for phrase list headers: Keywords
r     + Keywords
    + GParametrized*   // Base Class for parametrizable headers: Content-type
      + GContentType*
m       + ContentType
      + GTokenWithParameterList*
m       + ContentDisposition

    + GDate*           // Base class for date headers: Date
r     + Date
    + GNameValPairs*   // Base Class for name-value list headers: Received
r     + Received


common interface:

bool     isValid()
bool     isEmpty()

const char* type();

Entity* parent();
void     setParent( Content* );


//
// input/output:
//

// 7bit:
TQCString as7BitString()
bool     from7BitString( const TQCString & ) // deep copy
bool     from7BitString( const char * begin, const char * end ) // shallow
bool     from7BitString( const char * begin, int length ) // shallow
bool     from7BitString( TQByteArray &, int pos, int len ) // shallow
TQStringList validate( const TQCString & str );


// for dialogs:
TQString asUnicodeString()
bool    fromUnicodeString()
TQStringList validate( const TQString & str );

// for reader display:
TQString asHtmlString()
bool    fromHtmlString() // ???