int offset = selectionStart - descendant.getDocumentOffset();
if (descendant instanceof TextNode) {
// The caret is positioned within an existing text node
TextNode textNode = (TextNode)descendant;
textNode.insertText(text, offset);
} else if (descendant instanceof Paragraph) {
// The caret is positioned on the paragraph terminator
Paragraph paragraph = (Paragraph)descendant;
int n = paragraph.getLength();