code: synchronized PageWrapper insert(PageWrapper page) {
          this.pageByOffset.put(page.getOffset(), page);
          return page;
      }
docs: Inserts a new PageWrapper into this PageCollection.
  
  @param page The PageWrapper to insert.
  @return The inserted PageWrapper.
  @throws IllegalArgumentException If this method was previously invoked with a PageWrapper having isNewPage() == true
  but complete() or remove() have not been called on that PageWrapper yet.