code: void setPage(Page page) {
  		synchronized(lock) {
  			checkNotFrozen();
  			if(this.page != null) throw new IllegalStateException("element already has a page: " + this);
  			this.page = page;
  		}
  		assert checkPageAndParentElement();
  	}
docs: This is set when the element is associated with the page.
  
  @see Page#addElement(com.aoindustries.docs.Element)