code: public ZoneRulesBuilder setFixedSavingsToWindow(int fixedSavingAmountSecs) {
          if (windowList.isEmpty()) {
              throw new IllegalStateException("Must add a window before setting the fixed savings");
          }
          TZWindow window = windowList.get(windowList.size() - 1);
          window.setFixedSavings(fixedSavingAmountSecs);
          return this;
      }
docs: Sets the previously added window to have fixed savings.
  <p>
  Setting a window to have fixed savings simply means that a single daylight
  savings amount applies throughout the window. The window could be small,
  such as a single summer, or large, such as a multi-year daylight savings.
  <p>
  A window can either have fixed savings or rules but not both.
  
  @param fixedSavingAmountSecs  the amount of saving to use for the whole window, not null
  @return this, for chaining
  @throws IllegalStateException if no window has yet been added
  @throws IllegalStateException if the window already has rules