// Create source info and add serialized form as attribute.
SourceInfo pframe = new SourceInfo(documentUrl,
(! blockStack.isEmpty()) ? blockStack.peekLast() : null,
sequenceCounter, locator.getLineNumber(), locator.getColumnNumber());
element.setAttribute("source-info", pframe.toJSONString());
// Increase sequence counter.
++sequenceCounter;
// Add and set current