Resolve topic fragments and code references (topic-fragment)
The topic-fragment step expands content references to elements in the same topic and
resolves references made with the <coderef> element. This step is implemented in SAX
pipes.
Content references to elements in the same topic are defined via same-topic fragments such as
#./ID in URIs.
The <coderef> element is used to reference code stored externally in non-XML documents.
During the pre-processing step, the referenced content is pulled into the containing
<codeblock> element.