New customizable variable, notmuch-tree-thread-symbols, that allows
tweaking how trees in a forest are represented.  For instance, one can
now choose to use an hyphen rather than a white space as a prefix, or
remove pointy arrows.
---
 emacs/notmuch-tree.el | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/emacs/notmuch-tree.el b/emacs/notmuch-tree.el
index 01a77b71..045b8845 100644
--- a/emacs/notmuch-tree.el
+++ b/emacs/notmuch-tree.el
@@ -74,6 +74,21 @@
       notmuch-unthreaded-show-out
     notmuch-tree-show-out))
 
+(defcustom notmuch-tree-thread-symbols '(" " "─" "┬" "│" "├" "╰" "►")
+  "List of strings used to draw trees in notmuch tree results.
+The first element is used at the top of the tree, followed by the
+second if it has no children or the third if it does.  The fourth
+is a bar connecting with a response down the list skipping the
+current one, while the fifth marks the current message as a reply
+to the previous.  The sixth string is used the bottom of the
+thread.  Finally, the last one is a pointer to every message.
+
+Common customizations include setting the first element of the
+list to \"-\", to see equal-length prefixes, or the last to an
+empty string or to a different kind of arrow point."
+  :type '(list string)
+  :group 'notmuch-tree)
+
 (defcustom notmuch-tree-result-format
   `(("date" . "%12s  ")
     ("authors" . "%-20s")
@@ -968,20 +983,20 @@ message together with all its descendents."
 	(replies (cadr tree)))
     (cond
      ((and (< 0 depth) (not last))
-      (push "├" tree-status))
+      (push (elt notmuch-tree-thread-symbols 4) tree-status))
      ((and (< 0 depth) last)
-      (push "╰" tree-status))
+      (push (elt notmuch-tree-thread-symbols 5) tree-status))
      ((and (eq 0 depth) first last)
-      ;; Choice between these two variants is a matter of taste.
-      ;; (push "─" tree-status))
-      (push " " tree-status))
+      (push (elt notmuch-tree-thread-symbols 0) tree-status))
      ((and (eq 0 depth) first (not last))
-      (push "┬" tree-status))
+      (push (elt notmuch-tree-thread-symbols 2) tree-status))
      ((and (eq 0 depth) (not first) last)
-      (push "╰" tree-status))
+      (push (elt notmuch-tree-thread-symbols 5) tree-status))
      ((and (eq 0 depth) (not first) (not last))
-      (push "├" tree-status)))
-    (push (concat (if replies "┬" "─") "►") tree-status)
+      (push (elt notmuch-tree-thread-symbols 4) tree-status)))
+    (push (concat (elt notmuch-tree-thread-symbols (if replies 2 1))
+		  (elt notmuch-tree-thread-symbols 6))
+	  tree-status)
     (setq msg (plist-put msg :first (and first (eq 0 depth))))
     (setq msg (plist-put msg :tree-status tree-status))
     (setq msg (plist-put msg :orig-tags (plist-get msg :tags)))
@@ -990,7 +1005,7 @@ message together with all its descendents."
     (pop tree-status)
     (if last
 	(push " " tree-status)
-      (push "│" tree-status))
+      (push (elt notmuch-tree-thread-symbols 3) tree-status))
     (notmuch-tree-insert-thread replies (1+ depth) tree-status)))
 
 (defun notmuch-tree-insert-thread (thread depth tree-status)
-- 
2.33.0
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