1.4 Creating and Updating XML 11
Figure 1.4 File management vs. XML document management. The image on the
left used the Windows file manager. It shows disk drives, folders, and files on a PC.
The image on the right used the Xerlin XML document editor. It shows the elements
of a single XML document.
<!ELEMENT molecule (atomArray,bondArray)>
<!ELEMENT atomArray (atom+)>
<!ELEMENT bondArray (bond+)>TheELEMENTstatements above determine thecontentof these elements. A
specification such as(atom+)is called a content model. TheATTLISTstate-
ment formoleculegiven earlier determines the attributes that can be in
an element. A DTD will normally have oneELEMENTstatement and one
ATTLISTstatement for each kind of element that can be in the document. A
more complete DTD for molecules is shown in figure 1.6. Because the same
attributes are allowed in many elements, a DTD can be very long.ENTITY
statements are a method for simplifying the writing of DTDs, by allowing
one to specify content and lists of attributes just once. In figure 1.7 two enti-
ties were defined and then used several times. Large DTDs such as CML use
a large number of entities. These are just two of the entities in CML.