An external module in the Archiving and Interchange DTD Suite.
Declares the elements in the Link Class; these are elements that are links (internal or external) by definition, such as URLs
(element <uri>) and cross-references (element <xref>).
<!ENTITY % link.ent PUBLIC "-//NLM//DTD Archiving and
Interchange DTD Suite Link Class Elements v1.0 20021201//EN" "link.ent"