threading, missing symbol to 'standalone' articles

[Nicolas KOWALSKI]

Hello,

When in the Summary buffer, I notice than articles without answers
have no symbol to the left of the subject, but those who have answers
have this symbols. Here is an example:

RN  09-Mar [Timothy Murphy    ] (5.3k)   LAN addresses in IPv6
O   07-Mar [Gene Heskett      ] (4.6k) -> F10, cups, amanda problem.
 N  09-Mar [Tim Waugh         ] (4.5k) `->
O   08-Mar [Clodoaldo Pinto Ne] (4.4k) -> F10 updates breaking things
O   09-Mar [Patrick O'Callagha] (5.0k) `->
O   09-Mar [Alan Cox          ] (3.8k)  `->
O   09-Mar [Michael Schwendt  ] (4.9k)   `->
 N  09-Mar [psmith            ] (5.3k)    `->
 N  09-Mar [Neal Becker       ] (3.7k) -> How to send DHCP hostname with NM?


The first article does not have the '->' symbol preprended to the
subject.

Is there a way to prepend this symbol to such articles?


My threading configuration is currently:

(setq
 gnus-thread-indent-level 1
 gnus-suppress-duplicates nil
 gnus-auto-select-first nil
 gnus-fetch-old-headers t
 gnus-build-sparse-threads nil
 gnus-show-threads t
 gnus-thread-hide-killed t
 gnus-thread-hide-subtree nil
 gnus-summary-thread-gathering-function 'gnus-gather-threads-by-references
 gnus-thread-sort-functions '((not gnus-thread-sort-by-most-recent-number))
 gnus-simplify-subject-functions '(gnus-simplify-subject-re 
gnus-simplify-whitespace)
)


;;
;; thread view
;;
(setq gnus-sum-thread-tree-root "->"
      gnus-sum-thread-tree-false-root "->"
      gnus-sum-thread-tree-single-indent " "
      gnus-sum-thread-tree-vertical "|"
      gnus-sum-thread-tree-indent " "
      gnus-sum-thread-tree-leaf-with-other "+-> "
      gnus-sum-thread-tree-single-leaf "`-> ")


Thanks,
-- 
Nicolas