Line numbers, currently unused, are meant for drawing brackets in
interactive viewer. I'm still unsure how they will be eventually used,
but right now they cause exceptions on empty block (notably, when you
press END against BEGIN, which is a natural things to do). Let's plug it
for now.
To my understanding, Raspberry Pi has only one antenna for 2.4 GHz and
it's shared between WiFi and Bluetooth. If WiFi is not rfkilled,
Bluetooth may have delays in transmissions, which results if robot
noticeably freezing. So add a warning to remind the operator to kill
WiFi.
