code: public boolean debugIsPageIdInFreeList(int pageId) {
          return (toAdd.contains(pageId) || idx.findValue(pageId) != null) 
          		&& !toDelete.contains(pageId);
      }
docs: Simply speaking, this returns {@code true} if the given pageId is considered free.
  Returns {@code true} if the given pageId is in the known (currently free) or newly freed
  (will be free after next commit) and has not been re-occupied yet.
  @param pageId The page ID to check
  @return Whether the given pageId refers to a free page