Block a user
The command stack should be optimized
I believe this is actually what's at issue behind the performance issues with this sketch: https://web.ludus.dev/?code=fn+mover+%28%29+-%3E+%7B%0A++receive+%7B%0A++++%22w%22+-%3E+forward%21+%280.01…
spawn_turtle
should not break stack discipline