Resolve map references (mapref)

The mapref step resolves references from one DITA map to another. This step is implemented in XSLT.

Maps reference other maps by using the following sorts of markup: 
<topicref href="other.ditamap" format="ditamap"/>
...
<mapref href="other.ditamap"/>

As a result of the mapref step, the element that references another map is replaced by the topic references from the other map. Relationship tables are pulled into the referencing map as a child of the root element (<map> or a specialization of <map>).