int end = i+(GROUP_SIZE-2);
int start = i; // natural order
// int start = end - 1; // reverse order
while (start < end) { // natural order
// while (start >= i) { // reverse order
sheet.groupRow(start, end);
//o_groupsNumber++;
boolean collapsed = isCollapsed();
//System.out.println("Set group " + start + "->" + end + " to " + collapsed);
sheet.setRowGroupCollapsed(start, collapsed);
start++; // natural order