docs: public static boolean checkOverlap (Service s1, Service s2) {
          if (s1.calendar == null || s2.calendar == null) {
              return false;
          }
          // overlap exists if at least one day of week is shared by two calendars
          boolean overlappingDays = s1.calendar.monday == 1 && s2.calendar.monday == 1 ||
                  s1.calendar.tuesday == 1 && s2.calendar.tuesday == 1 ||
                  s1.calendar.wednesday == 1 && s2.calendar.wednesday == 1 ||
                  s1.calendar.thursday == 1 && s2.calendar.thursday == 1 ||
                  s1.calendar.friday == 1 && s2.calendar.friday == 1 ||
                  s1.calendar.saturday == 1 && s2.calendar.saturday == 1 ||
                  s1.calendar.sunday == 1 && s2.calendar.sunday == 1;
          return overlappingDays;
      }