How strongly is Pyret committed to the JavaScript runtime? I read jpolitz's recent comment about the contortions that you have to go through to support recursion while not overflowing the stack or blocking the UI thread, and I wonder if it would really be so bad to require people to install a programming environment on their computers. How common is it really for students to be limited to what they can run in a browser? Anyway, have you considered a non-JS runtime of some kind?