static ItIt<Pt> allSubRectsOfSizeIterator_topLeftCorners(int w, int h, Rect full) {
  if (full == null) ret emptyItIt();
  int fullW = full.w, fullH = full.h;
  if (w > fullW || h > fullH) ret emptyItIt();
  int freeplayX = fullW-w+1, freeplayY = fullH-h+1, freeplay = freeplayX*freeplayY;
  ret mapI(iota_zero(freeplay), i ->
    pt(full.x + (i % freeplayX), full.y + (i / freeplayX)));
}

Began life as a copy of #1027216