private void fastInfosetManuallyAddPrefixMapping(final String prefix, final String uri) throws SAXException {
        LOGGER.trace("Manually starting prefix mapping {}:{}", prefix, uri);
        manuallyAddedLevelToPrefixMap.computeIfAbsent(depthLevel, key -> new HashSet<>())
                .add(prefix);
        fastInfosetStartPrefixMapping(prefix, uri);
    }
@param ch     An array of characters.
@param start  The starting position in the array.
@param length The number of characters to use from the array.
@throws SAXException The client may throw an exception during processing.
@see stroom.pipeline.filter.AbstractXMLFilter#characters(char[],
int, int)
stroom-state/stroom-state-impl/src/main/java/stroom/state/impl/pipeline/StateFilter.java