scope renderLineCompedSingleLevels.

sS renderLineCompedSingleLevels(LineCompedSingle<Char> lc) {
  int n = lcLength(lc);
  
  // each byte is the character's "color"
  // (0 for empty, 1 for color one, 2 for color two)
  L<byte[]> rows = new AutoGrowingList<>(() -> new byte[n]);
  fill(lc, rows, 0, 0, lc.main, 1);
  ret lines(buf);
}
  
// returns new x
static int #fill(LineCompedSingle<Char> lc, L<byte[] rows, int x, int y, L<Int> symbols, int color) {
  for (int idx : symbols) {
    if (idx < l(lc.literals)) {
      // nothing to do?
      ret x+1;
    } else {
      int newX = fill(lc, rows, x, y+1, idx, color);
      while (x < newX) rows.get(y)[x++] = color;
      color = 3-color;
    }
  }
  ret x;
}