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14 1 Hierarchies and Relationships

’dictRef CDATA #IMPLIED’>

<!ELEMENT molecule (atomArray, bondArray)>

<!ATTLIST molecule

%title_id_conv_dict;

count CDATA "1"

>

<!ELEMENT atomArray (atom+)>

<!ATTLIST atomArray

%title_id_conv;

>

<!ELEMENT atom EMPTY>

<!ATTLIST atom

elementType CDATA #IMPLIED

%title_id_conv_dict;

count CDATA "1"

>

<!ELEMENT bondArray (bond+)>

<!ATTLIST bondArray

%title_id_conv;

>

<!ELEMENT bond EMPTY>

<!ATTLIST bond

%title_id_conv_dict;

dictRef CDATA #IMPLIED

atomRefs CDATA #IMPLIED

>

Figure 1.7 Part of the Chemical Markup Language DTD. This DTD uses entities to

simplify the DTD in figure 1.6.

In addition to simplifying a DTD, there are other uses of XML entities:

• Entities can be used to build a large DTD from smaller files. The entities
  in this case refer to the files being incorporated rather than to the actual
  value of the entity. Such an entity would be defined like this: