Sorted subpillar change/swap selector which works wonders on Nurse Rostering and Airline crew scheduling

say a pillar for ann has these shifts:

• Mon
• Tue
• Wed
• Thu
• Fri

Randomly selecting a subset of those is not ideal in some cases.
It's far better to select a sorted sequence subset, so for example Mon-Tue-Wed or Tue-Wed or Tue-Wed-Thu-Fri.