Attribute declarations

<!ATTLIST poem 
          id     ID                        #IMPLIED
          status (draft|revised|published) draft   >

In document: <poem id=P1 status=revised> ... </poem>

Some possible declared values:

• CDATA: any valid character data;
• SDATA: system dependent data;
• NUMBER: composed only of numerals;
• ID: a unique identifier, composed of any valid character data
• IDREF: a pointer to an ID

Some possible keywords:

• #REQUIRED: a value must be specified;
• #IMPLIED: a value need not be supplied;
• #CURRENT: if no value is supplied, the last specified value should be used.