Thanks to help received in the forum, I've now managed to solve a problem that was bothering me in Serendipity, viz. how to add more than one category to a posting. It's actually quite simple. You first click the + sign next to the category box, which changes it to a -minus sign. Then you select the categories you want, using the mouse and the Ctrl key (this is important). Quite easy, really, but not intuitively obvious.