@Override
    public void startPrefixMapping(final String prefix, final String uri) throws SAXException {
        contentHandler.startPrefixMapping(prefix, uri);
    }
Begin the scope of a prefix-URI Namespace mapping.
<p>
<p>
The information from this event is not necessary for normal Namespace
processing: the SAX XML reader will automatically replace prefixes for
element and attribute names when the
<code>http://xml.org/sax/features/namespaces</code> feature is
<var>true</var> (the default).
</p>
<p>
<p>
There are cases, however, when applications need to use prefixes in
character data or in attribute values, where they cannot safely be
expanded automatically; the start/endPrefixMapping event supplies the
information to the application to expand prefixes in those contexts
itself, if necessary.
</p>
<p>
<p>
Note that start/endPrefixMapping events are not guaranteed to be properly
nested relative to each other: all startPrefixMapping events will occur
immediately before the corresponding {@link #startElement startElement}
event, and all {@link #endPrefixMapping endPrefixMapping} events will
occur immediately after the corresponding {@link #endElement endElement}
event, but their order is not otherwise guaranteed.
</p>
<p>
<p>
There should never be start/endPrefixMapping events for the "xml" prefix,
since it is predeclared and immutable.
</p>

@param prefix the Namespace prefix being declared. An empty string is used
              for the default element namespace, which has no prefix.
@param uri    the Namespace URI the prefix is mapped to
@throws org.xml.sax.SAXException the client may throw an exception during processing
@see #endPrefixMapping
@see #startElement
stroom-pipeline/src/main/java/stroom/pipeline/filter/AbstractXMLFilter.java