Thanks. From memory we were using repl.it, or something very similar. This made it easy for me to look at their code when they had problems (and even then, as I said to someone else, it was quite a while before I realised one of them simply had the wrong indentation on one line - I kept looking at the code and thinking I couldn't see anything wrong with it, then eventually I realised there was a wrong indentation. If it took me that long to realise, then of course that's something students are going to struggle with).