code: private Long storeTree(MtasTree<?> tree, IndexOutput out,
        long refApproxOffset) throws IOException {
      return storeTree(tree.close(), tree.isSinglePoint(),
          tree.isStorePrefixAndTermRef(), out, null, refApproxOffset);
    }
docs: Store tree.
  
  @param tree
  the tree
  @param out
  the out
  @param refApproxOffset
  the ref approx offset
  @return the long
  @throws IOException
  Signals that an I/O exception has occurred.