@Override
    public void characters(final char[] ch, final int start, final int length) throws SAXException {
        if (depth > 0) {
            super.characters(ch, start, length);
        }
    }
@param ch     the characters from the XML document
@param start  the start position in the array
@param length the number of characters to read from the array
@throws org.xml.sax.SAXException any SAX exception, possibly wrapping another exception
@see #ignorableWhitespace
@see org.xml.sax.Locator
@see stroom.pipeline.filter.AbstractXMLFilter#characters(char[],
int, int)
stroom-pipeline/src/main/java/stroom/pipeline/filter/MergeFilter.java