snapshot

It took me _way_ too long to realize that I'm not checking for errors within
the loop, and that will cause it to manifest as an infinite loop as inner
evaluations fail to run.

Debugging notes, for posterity:
  printing one row of a chessboard pattern over fake screen (chessboard screen 4 0 0 15) gets stuck in an infinite loop halfway through
    debug pattern during infinite loop: VWEX. It's still in the loop but it's not executing the body
    raw (fill_rect screen 16 0 20 4 15) works fine
    same number of calls to fill_rect work fine
    replacing calls to fill_rect with pixel inside chessboard2 works fine
    at the point of the infinite loop it's repeatedly going through the hline loop
      -- BUT it never executes the check of the loop (< lo hi) with lo=20, hi=20. Something is returning 1, but it's not inside <
    stream optimization is not implicated

  simple test case with a single loop
    (
      (globals . (
        (foo . (fn () (screen i n)
                 (while (< i n)
                   (pixel screen 4 4 i)
                   (pixel screen 5 4 i)
                   (pixel screen 6 4 i)
                   (pixel screen 7 4 i)
                   (set i (+ i 1)))))
      ))
      (sandbox . (foo screen 0 100))
    )

  simpler (if you reset cursor position before every print):
    (
      (globals . (
        (foo . (fn () (screen i n)
                 (while (< i n)
                   (print screen i)
                   (set i (+ i 1)))))
      ))
      (sandbox . (foo screen 0 210))
    )

  I now believe it has nothing to do with the check. The check always works.
  Sometimes no body is evaluated. And so the set has no effect.

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