@Override
    public void setDocumentLocator(final Locator locator) {
        if (this.locator == null) {
            this.locator = locator;
            super.setDocumentLocator(locator);
        }
    }
@param locator an object that can return the location of any SAX document
               event
@see org.xml.sax.Locator
@see stroom.pipeline.filter.AbstractXMLFilter#setDocumentLocator(org.xml.sax.Locator)
stroom-pipeline/src/main/java/stroom/pipeline/filter/XsltFilter.java