code: private void addWindowManager(WindowManager wm) {
          if (subManagers == null) {
              subManagers = new ArrayList();
          }
          if (!subManagers.contains(wm)) {
              subManagers.add(wm);
              wm.parentManager = this;
          }
      }
docs: Adds the given window manager to the list of window managers that have
  this one as a parent.
  
  @param wm
  the child window manager