Basic inter-document and external linking support.

docs/dtd/module.dtd

diff --git a/docs/dtd/module.dtd b/docs/dtd/module.dtd
index 0b21673..d280b90 100644
--- a/docs/dtd/module.dtd
+++ b/docs/dtd/module.dtd
@@ -55,6 +55,7 @@
 <!ATTLIST   link
             doc         CDATA #IMPLIED
             id          CDATA #IMPLIED
+            url         CDATA #IMPLIED
 >
 
 <!ELEMENT   list        (listitem+ | (tag-name | tag-desc)+) >

docs/xsls/content.xsls

diff --git a/docs/xsls/content.xsls b/docs/xsls/content.xsls
index 9d4bd02..c6fe14e 100644
--- a/docs/xsls/content.xsls
+++ b/docs/xsls/content.xsls
@@ -39,7 +39,14 @@
         X:text{ (} !{@code} X:text{)}
     }
 
+    X:template = "link[@url]" { <a href="{@url}"> !!; </a> }
     X:template = "link[@id and not(@doc)]" { <a href="#{@id}"> !!; </a> }
+    X:template = "link[@doc and not(@id)]" {
+        <a href="{substring-before(@doc, '.xml')}.html"> !!; </a>
+    }
+    X:template = "link[@id and @doc]" {
+        <a href="{substring-before(@doc, '.xml')}.html#{@id}"> !!; </a>
+    }
     X:template = "link" { <u> !!; </u> }
 
     X:template = "list[@type='bullet']" { <ul> !!; </ul> }