docs: private HeaderElement getElement(HeaderKeys key) {
          HeaderElement elem = this.headerElements;
          if (null != elem) {
              // disconnect it from the rest of the free list
              this.headerElements = elem.nextInstance;
              elem.nextInstance = null;
              elem.init(key);
          } else {
              elem = new HeaderElement(key, this);
          }
          return elem;
      }