static L<Rect> ocr_groupRects(L<Rect> rects, S s) {
  L<S> l = ocr_parseGlyphs(dropSpaces(s));
  assertEquals(l(rects), l(l));
  new L<Rect> rects2;
  for (int i = 0; i < l(l); ) {
    int j = i+1;
    while (j < l(l) && eq(l.get(j), "_")) ++j;
    rects2.add(mergeRects(subList(rects, i, j)));
    i = j;
  }
  ret rects2;
}