code: private short addTrackedHeader(final short index, final SipHeader header) {
          if (index != -1) {
              headers.set(index, header.ensure());
              return index;
          }
          return addHeader(header.ensure());
      }
docs: There are several headers that we want to know their position
  and in that case we use this method to add them since we want
  to either add them to a particular position or we want to
  remember which index we added them to.