function addCells (row, tidx, tend, textLines, change) {
			if (tidx < tend) {
				row.appendChild(telt("th", (tidx + 1).toString()));
				row.appendChild(ctelt("td", change, textLines[tidx].replace(/\t/g, "\u00a0\u00a0\u00a0\u00a0")));
				return tidx + 1;
			} else {
				row.appendChild(document.createElement("th"));
				row.appendChild(celt("td", "empty"));
				return tidx;
			}
		}
Adds two cells to the given row; if the given row corresponds to a real
line number (based on the line index tidx and the endpoint of the 
range in question tend), then the cells will contain the line number
and the line of text from textLines at position tidx (with the class of
the second cell set to the name of the change represented), and tidx + 1 will
be returned.	 Otherwise, tidx is returned, and two empty cells are added
to the given row.