>From 380c05f87fac0fb62ed6732729e865e1ff375583 Mon Sep 17 00:00:00 2001
From: Alex Branham <alex.branham@gmail.com>
Date: Mon, 10 Dec 2018 13:19:04 -0600
Subject: [PATCH] New function flatten-tree

* lisp/subr.el (flatten-tree): New defun

* lisp/subr.el (flatten-list): Alias to `flatten-tree' for discoverability

* lisp/progmodes/js.el (js--flatten-list):
* lisp/lpr.el (lpr-flatten-list):
* lisp/gnus/message.el (message-flatten-list):
* lisp/eshell/esh-util.el (eshell-flatten-list):
Obsolete in favor of Emacs-wide `flatten-tree'

* lisp/progmodes/js.el (js--maybe-join):
* lisp/printing.el (pr-switches):
* lisp/lpr.el (lpr-print-region):
* lisp/gnus/nnimap.el (nnimap-find-wanted-parts):
* lisp/gnus/message.el (message-talkative-question):
* lisp/gnus/gnus-sum.el (gnus-remove-thread)
(gnus-thread-highest-number, gnus-thread-latest-date):
* lisp/eshell/esh-util.el (eshell-flatten-and-stringify):
* lisp/eshell/esh-opt.el (eshell-eval-using-options):
* lisp/eshell/esh-ext.el (eshell-external-command):
* lisp/eshell/em-xtra.el (eshell/expr):
* lisp/eshell/em-unix.el (eshell/rm, eshell-mvcpln-template)
(eshell/cat, eshell/make, eshell-poor-mans-grep, eshell-grep)
(eshell/du, eshell/time, eshell/diff, eshell/locate):
* lisp/eshell/em-tramp.el (eshell/su, eshell/sudo):
* lisp/eshell/em-term.el (eshell-exec-visual):
* lisp/eshell/em-dirs.el (eshell-dirs-substitute-cd, eshell/cd):
* lisp/eshell/em-basic.el (eshell/printnl):
Use new flatten-tree

* doc/lispref/lists.texi: Document `flatten-tree'

Co-authored-by: Basil L. Contovounesios <contovob@tcd.ie>
Bug #33309
---
 doc/lispref/lists.texi  | 12 ++++++++++++
 etc/NEWS                |  6 ++++++
 lisp/eshell/em-basic.el |  2 +-
 lisp/eshell/em-dirs.el  |  4 ++--
 lisp/eshell/em-term.el  |  2 +-
 lisp/eshell/em-tramp.el |  4 ++--
 lisp/eshell/em-unix.el  | 22 +++++++++++-----------
 lisp/eshell/em-xtra.el  |  2 +-
 lisp/eshell/esh-ext.el  |  2 +-
 lisp/eshell/esh-opt.el  |  4 ++--
 lisp/eshell/esh-util.el | 12 ++----------
 lisp/gnus/gnus-sum.el   | 10 +++++-----
 lisp/gnus/message.el    | 12 ++----------
 lisp/gnus/nnimap.el     |  2 +-
 lisp/lpr.el             | 20 ++------------------
 lisp/printing.el        |  2 +-
 lisp/progmodes/js.el    |  8 ++------
 lisp/subr.el            | 25 +++++++++++++++++++++++++
 test/lisp/subr-tests.el | 17 +++++++++++++++++
 19 files changed, 96 insertions(+), 72 deletions(-)

diff --git a/doc/lispref/lists.texi b/doc/lispref/lists.texi
index 1548dd49b2..52e3c312d9 100644
--- a/doc/lispref/lists.texi
+++ b/doc/lispref/lists.texi
@@ -667,6 +667,18 @@ Building Lists
 their elements).
 @end defun
 
+@defun flatten-tree tree
+Take @var{tree} and "flatten" it.
+This always returns a list containing all the terminal nodes, or
+leaves, of @var{tree}.  Dotted pairs are flattened as well, and nil
+elements are removed.
+@end defun
+
+@example
+(flatten-tree '(1 (2 . 3) nil (4 5 (6)) 7))
+    @result{}(1 2 3 4 5 6 7)
+@end example
+
 @defun number-sequence from &optional to separation
 This returns a list of numbers starting with @var{from} and
 incrementing by @var{separation}, and ending at or just before
diff --git a/etc/NEWS b/etc/NEWS
index 6ae994d594..0caf123e2d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1343,6 +1343,12 @@ are implemented in C using the Jansson library.
 ** New function 'ring-resize'.
 'ring-resize' can be used to grow or shrink a ring.
 
++++
+** New function 'flatten-tree'.
+'flatten-list' is provided as an alias. These functions take a tree
+and 'flatten' it such that the result is a list of all the terminal
+nodes.
+
 ** Mailcap
 
 ---
diff --git a/lisp/eshell/em-basic.el b/lisp/eshell/em-basic.el
index 5201076f48..4a99d83857 100644
--- a/lisp/eshell/em-basic.el
+++ b/lisp/eshell/em-basic.el
@@ -118,7 +118,7 @@ eshell/echo
 
 (defun eshell/printnl (&rest args)
   "Print out each of the arguments, separated by newlines."
-  (let ((elems (eshell-flatten-list args)))
+  (let ((elems (flatten-tree args)))
     (while elems
       (eshell-printn (eshell-echo (list (car elems))))
       (setq elems (cdr elems)))))
diff --git a/lisp/eshell/em-dirs.el b/lisp/eshell/em-dirs.el
index 853382888c..b47f76fbfb 100644
--- a/lisp/eshell/em-dirs.el
+++ b/lisp/eshell/em-dirs.el
@@ -259,7 +259,7 @@ eshell-dirs-substitute-cd
   (if (> (length args) 1)
       (error "%s: command not found" (car args))
     (throw 'eshell-replace-command
-	   (eshell-parse-command "cd" (eshell-flatten-list args)))))
+	   (eshell-parse-command "cd" (flatten-tree args)))))
 
 (defun eshell-parse-user-reference ()
   "An argument beginning with ~ is a filename to be expanded."
@@ -353,7 +353,7 @@ eshell-find-previous-directory
 
 (defun eshell/cd (&rest args)           ; all but first ignored
   "Alias to extend the behavior of `cd'."
-  (setq args (eshell-flatten-list args))
+  (setq args (flatten-tree args))
   (let ((path (car args))
 	(subpath (car (cdr args)))
 	(case-fold-search (eshell-under-windows-p))
diff --git a/lisp/eshell/em-term.el b/lisp/eshell/em-term.el
index ddde47f73d..fdf40cae85 100644
--- a/lisp/eshell/em-term.el
+++ b/lisp/eshell/em-term.el
@@ -175,7 +175,7 @@ eshell-exec-visual
   (let* (eshell-interpreter-alist
 	 (interp (eshell-find-interpreter (car args) (cdr args)))
 	 (program (car interp))
-	 (args (eshell-flatten-list
+	 (args (flatten-tree
 		(eshell-stringify-list (append (cdr interp)
 					       (cdr args)))))
 	 (term-buf
diff --git a/lisp/eshell/em-tramp.el b/lisp/eshell/em-tramp.el
index 9475f4ed94..f77b84d851 100644
--- a/lisp/eshell/em-tramp.el
+++ b/lisp/eshell/em-tramp.el
@@ -62,7 +62,7 @@ eshell/su
   "Alias \"su\" to call TRAMP.
 
 Uses the system su through TRAMP's su method."
-  (setq args (eshell-stringify-list (eshell-flatten-list args)))
+  (setq args (eshell-stringify-list (flatten-tree args)))
   (let ((orig-args (copy-tree args)))
     (eshell-eval-using-options
      "su" args
@@ -100,7 +100,7 @@ eshell/sudo
   "Alias \"sudo\" to call Tramp.
 
 Uses the system sudo through TRAMP's sudo method."
-  (setq args (eshell-stringify-list (eshell-flatten-list args)))
+  (setq args (eshell-stringify-list (flatten-tree args)))
   (let ((orig-args (copy-tree args)))
     (eshell-eval-using-options
      "sudo" args
diff --git a/lisp/eshell/em-unix.el b/lisp/eshell/em-unix.el
index 3aecebc2eb..e46e1c417d 100644
--- a/lisp/eshell/em-unix.el
+++ b/lisp/eshell/em-unix.el
@@ -231,7 +231,7 @@ eshell/rm
 This is implemented to call either `delete-file', `kill-buffer',
 `kill-process', or `unintern', depending on the nature of the
 argument."
-  (setq args (eshell-flatten-list args))
+  (setq args (flatten-tree args))
   (eshell-eval-using-options
    "rm" args
    '((?h "help" nil nil "show this usage screen")
@@ -481,7 +481,7 @@ eshell-mvcpln-template
 	 (error "%s: missing destination file or directory" ,command))
      (if (= len 1)
 	 (nconc args '(".")))
-     (setq args (eshell-stringify-list (eshell-flatten-list args)))
+     (setq args (eshell-stringify-list (flatten-tree args)))
      (if (and ,(not (equal command "ln"))
 	      (string-match eshell-tar-regexp (car (last args)))
 	      (or (> (length args) 2)
@@ -606,7 +606,7 @@ eshell/cat
   "Implementation of cat in Lisp.
 If in a pipeline, or the file is not a regular file, directory or
 symlink, then revert to the system's definition of cat."
-  (setq args (eshell-stringify-list (eshell-flatten-list args)))
+  (setq args (eshell-stringify-list (flatten-tree args)))
   (if (or eshell-in-pipeline-p
 	  (catch 'special
 	    (dolist (arg args)
@@ -670,7 +670,7 @@ eshell/make
 	(compile (concat "make " (eshell-flatten-and-stringify args))))
     (throw 'eshell-replace-command
 	   (eshell-parse-command "*make" (eshell-stringify-list
-					  (eshell-flatten-list args))))))
+					  (flatten-tree args))))))
 
 (put 'eshell/make 'eshell-no-numeric-conversions t)
 
@@ -705,7 +705,7 @@ eshell-poor-mans-grep
 	  (erase-buffer)
 	  (occur-mode)
 	  (let ((files (eshell-stringify-list
-			(eshell-flatten-list (cdr args))))
+			(flatten-tree (cdr args))))
 		(inhibit-redisplay t)
 		string)
 	    (when (car args)
@@ -750,11 +750,11 @@ eshell-grep
 	(throw 'eshell-replace-command
 	       (eshell-parse-command (concat "*" command)
 				     (eshell-stringify-list
-				      (eshell-flatten-list args))))
+				      (flatten-tree args))))
       (let* ((args (mapconcat 'identity
 			      (mapcar 'shell-quote-argument
 				      (eshell-stringify-list
-				       (eshell-flatten-list args)))
+				       (flatten-tree args)))
 			      " "))
 	     (cmd (progn
 		    (set-text-properties 0 (length args)
@@ -876,7 +876,7 @@ eshell-du-sum-directory
 (defun eshell/du (&rest args)
   "Implementation of \"du\" in Lisp, passing ARGS."
   (setq args (if args
-		 (eshell-stringify-list (eshell-flatten-list args))
+		 (eshell-stringify-list (flatten-tree args))
 	       '(".")))
   (let ((ext-du (eshell-search-path "du")))
     (if (and ext-du
@@ -976,7 +976,7 @@ eshell/time
 	    (eshell-parse-command (car time-args)
 ;;; https://lists.gnu.org/r/bug-gnu-emacs/2007-08/msg00205.html
 				  (eshell-stringify-list
-				   (eshell-flatten-list (cdr time-args))))))))
+				   (flatten-tree (cdr time-args))))))))
 
 (defun eshell/whoami (&rest _args)
   "Make \"whoami\" Tramp aware."
@@ -1000,7 +1000,7 @@ nil-blank-string
 
 (defun eshell/diff (&rest args)
   "Alias \"diff\" to call Emacs `diff' function."
-  (let ((orig-args (eshell-stringify-list (eshell-flatten-list args))))
+  (let ((orig-args (eshell-stringify-list (flatten-tree args))))
     (if (or eshell-plain-diff-behavior
 	    (not (and (eshell-interactive-output-p)
 		      (not eshell-in-pipeline-p)
@@ -1056,7 +1056,7 @@ eshell/locate
 	       (string-match "^-" (car args))))
       (throw 'eshell-replace-command
 	     (eshell-parse-command "*locate" (eshell-stringify-list
-					      (eshell-flatten-list args))))
+					      (flatten-tree args))))
     (save-selected-window
       (let ((locate-history-list (list (car args))))
 	(locate-with-filter (car args) (cadr args))))))
diff --git a/lisp/eshell/em-xtra.el b/lisp/eshell/em-xtra.el
index cc84d19854..eb9847c60c 100644
--- a/lisp/eshell/em-xtra.el
+++ b/lisp/eshell/em-xtra.el
@@ -51,7 +51,7 @@ eshell/expr
   "Implementation of expr, using the calc package."
   (if (not (fboundp 'calc-eval))
       (throw 'eshell-replace-command
-	     (eshell-parse-command "*expr" (eshell-flatten-list args)))
+	     (eshell-parse-command "*expr" (flatten-tree args)))
     ;; to fool the byte-compiler...
     (let ((func 'calc-eval))
       (funcall func (eshell-flatten-and-stringify args)))))
diff --git a/lisp/eshell/esh-ext.el b/lisp/eshell/esh-ext.el
index 244cc7ff1f..9e7d8bb608 100644
--- a/lisp/eshell/esh-ext.el
+++ b/lisp/eshell/esh-ext.el
@@ -222,7 +222,7 @@ eshell-remote-command
 
 (defun eshell-external-command (command args)
   "Insert output from an external COMMAND, using ARGS."
-  (setq args (eshell-stringify-list (eshell-flatten-list args)))
+  (setq args (eshell-stringify-list (flatten-tree args)))
   (let ((interp (eshell-find-interpreter
 		 command
 		 args
diff --git a/lisp/eshell/esh-opt.el b/lisp/eshell/esh-opt.el
index d7a449450f..69d10b4ccf 100644
--- a/lisp/eshell/esh-opt.el
+++ b/lisp/eshell/esh-opt.el
@@ -77,7 +77,7 @@ eshell-eval-using-options
   arguments.
 
 :preserve-args
-  If present, do not pass MACRO-ARGS through `eshell-flatten-list'
+  If present, do not pass MACRO-ARGS through `flatten-tree'
 and `eshell-stringify-list'.
 
 :parse-leading-options-only
@@ -106,7 +106,7 @@ eshell-eval-using-options
            ,(if (memq ':preserve-args (cadr options))
                 macro-args
               (list 'eshell-stringify-list
-                    (list 'eshell-flatten-list macro-args))))
+                    (list 'flatten-tree macro-args))))
           (processed-args (eshell--do-opts ,name ,options temp-args))
           ,@(delete-dups
              (delq nil (mapcar (lambda (opt)
diff --git a/lisp/eshell/esh-util.el b/lisp/eshell/esh-util.el
index 8fe8c461fd..b55f873380 100644
--- a/lisp/eshell/esh-util.el
+++ b/lisp/eshell/esh-util.el
@@ -285,15 +285,7 @@ eshell-for
 	 ,@forms)
        (setq list-iter (cdr list-iter)))))
 
-(defun eshell-flatten-list (args)
-  "Flatten any lists within ARGS, so that there are no sublists."
-  (let ((new-list (list t)))
-    (dolist (a args)
-      (if (and (listp a)
-	       (listp (cdr a)))
-	  (nconc new-list (eshell-flatten-list a))
-	(nconc new-list (list a))))
-    (cdr new-list)))
+(define-obsolete-function-alias 'eshell-flatten-list #'flatten-tree "27.1")
 
 (defun eshell-uniquify-list (l)
   "Remove occurring multiples in L.  You probably want to sort first."
@@ -330,7 +322,7 @@ eshell-stringify-list
 
 (defsubst eshell-flatten-and-stringify (&rest args)
   "Flatten and stringify all of the ARGS into a single string."
-  (mapconcat 'eshell-stringify (eshell-flatten-list args) " "))
+  (mapconcat 'eshell-stringify (flatten-tree args) " "))
 
 (defsubst eshell-directory-files (regexp &optional directory)
   "Return a list of files in the given DIRECTORY matching REGEXP."
diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index 4baf4bc826..3f5362ba17 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -4773,7 +4773,7 @@ gnus-remove-thread
   (let (headers thread last-id)
     ;; First go up in this thread until we find the root.
     (setq last-id (gnus-root-id id)
-	  headers (message-flatten-list (gnus-id-to-thread last-id)))
+	  headers (flatten-tree (gnus-id-to-thread last-id)))
     ;; We have now found the real root of this thread.  It might have
     ;; been gathered into some loose thread, so we have to search
     ;; through the threads to find the thread we wanted.
@@ -5069,7 +5069,7 @@ gnus-thread-highest-number
   "Return the highest article number in THREAD."
   (apply 'max (mapcar (lambda (header)
 			(mail-header-number header))
-		      (message-flatten-list thread))))
+		      (flatten-tree thread))))
 
 (defun gnus-article-sort-by-most-recent-date (h1 h2)
   "Sort articles by number."
@@ -5087,9 +5087,9 @@ gnus-thread-latest-date
   "Return the highest article date in THREAD."
   (apply 'max
 	 (mapcar (lambda (header) (float-time
-				   (gnus-date-get-time
-				    (mail-header-date header))))
-		 (message-flatten-list thread))))
+			      (gnus-date-get-time
+			       (mail-header-date header))))
+		 (flatten-tree thread))))
 
 (defun gnus-thread-total-score-1 (root)
   ;; This function find the total score of the thread below ROOT.
diff --git a/lisp/gnus/message.el b/lisp/gnus/message.el
index fdaa4e8272..03f80616d9 100644
--- a/lisp/gnus/message.el
+++ b/lisp/gnus/message.el
@@ -8051,7 +8051,7 @@ message-talkative-question
 If SHOW is non-nil, the arguments TEXT... are displayed in a temp buffer.
 The following arguments may contain lists of values."
   (if (and show
-	   (setq text (message-flatten-list text)))
+	   (setq text (flatten-tree text)))
       (save-window-excursion
         (with-output-to-temp-buffer " *MESSAGE information message*"
           (with-current-buffer " *MESSAGE information message*"
@@ -8061,15 +8061,7 @@ message-talkative-question
 	(funcall ask question))
     (funcall ask question)))
 
-(defun message-flatten-list (list)
-  "Return a new, flat list that contains all elements of LIST.
-
-\(message-flatten-list \\='(1 (2 3 (4 5 (6))) 7))
-=> (1 2 3 4 5 6 7)"
-  (cond ((consp list)
-	 (apply 'append (mapcar 'message-flatten-list list)))
-	(list
-	 (list list))))
+(define-obsolete-function-alias 'message-flatten-list #'flatten-tree "27.1")
 
 (defun message-generate-new-buffer-clone-locals (name &optional varstr)
   "Create and return a buffer with name based on NAME using `generate-new-buffer'.
diff --git a/lisp/gnus/nnimap.el b/lisp/gnus/nnimap.el
index 1a3b05ddb3..adbce25530 100644
--- a/lisp/gnus/nnimap.el
+++ b/lisp/gnus/nnimap.el
@@ -804,7 +804,7 @@ nnimap-insert-partial-structure
     (insert "\n--" boundary "--\n")))
 
 (defun nnimap-find-wanted-parts (structure)
-  (message-flatten-list (nnimap-find-wanted-parts-1 structure "")))
+  (flatten-tree (nnimap-find-wanted-parts-1 structure "")))
 
 (defun nnimap-find-wanted-parts-1 (structure prefix)
   (let ((num 1)
diff --git a/lisp/lpr.el b/lisp/lpr.el
index 33b8da8d76..969b57d644 100644
--- a/lisp/lpr.el
+++ b/lisp/lpr.el
@@ -258,7 +258,7 @@ print-region-1
 
 (defun lpr-print-region (start end switches name)
   (let ((buf (current-buffer))
-        (nswitches (lpr-flatten-list
+        (nswitches (flatten-tree
                     (mapcar #'lpr-eval-switch ; Dynamic evaluation
                             switches)))
         (switch-string (if switches
@@ -336,23 +336,7 @@ lpr-eval-switch
 	((consp arg) (apply (car arg) (cdr arg)))
 	(t nil)))
 
-;; `lpr-flatten-list' is defined here (copied from "message.el" and
-;; enhanced to handle dotted pairs as well) until we can get some
-;; sensible autoloads, or `flatten-list' gets put somewhere decent.
-
-;; (lpr-flatten-list '((a . b) c (d . e) (f g h) i . j))
-;; => (a b c d e f g h i j)
-
-(defun lpr-flatten-list (&rest list)
-  (lpr-flatten-list-1 list))
-
-(defun lpr-flatten-list-1 (list)
-  (cond
-   ((null list) nil)
-   ((consp list)
-    (append (lpr-flatten-list-1 (car list))
-	    (lpr-flatten-list-1 (cdr list))))
-   (t (list list))))
+(define-obsolete-function-alias 'lpr-flatten-list #'flatten-tree "27.1")
 
 (provide 'lpr)
 
diff --git a/lisp/printing.el b/lisp/printing.el
index 2fc2323028..c1a73df14c 100644
--- a/lisp/printing.el
+++ b/lisp/printing.el
@@ -5672,7 +5672,7 @@ pr-switches-string
 (defun pr-switches (switches mess)
   (or (listp switches)
       (error "%S should have a list of strings" mess))
-  (lpr-flatten-list			; dynamic evaluation
+  (flatten-tree			; dynamic evaluation
    (mapcar #'lpr-eval-switch switches)))
 
 
diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index cec48a82a2..ddba7636b4 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -623,11 +623,7 @@ js--state-at-last-parse-pos
   "Parse state at `js--last-parse-pos'.")
 (make-variable-buffer-local 'js--state-at-last-parse-pos)
 
-(defun js--flatten-list (list)
-  (cl-loop for item in list
-           nconc (cond ((consp item)
-                        (js--flatten-list item))
-                       (item (list item)))))
+(define-obsolete-function-alias 'js--flatten-list #'flatten-tree "27.1")
 
 (defun js--maybe-join (prefix separator suffix &rest list)
   "Helper function for `js--update-quick-match-re'.
@@ -636,7 +632,7 @@ js--maybe-join
 with SUFFIX as with `concat'.  Otherwise, if LIST is empty, return
 nil.  If any element in LIST is itself a list, flatten that
 element."
-  (setq list (js--flatten-list list))
+  (setq list (flatten-tree list))
   (when list
     (concat prefix (mapconcat #'identity list separator) suffix)))
 
diff --git a/lisp/subr.el b/lisp/subr.el
index d3bc007293..7a7c175db4 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -5448,5 +5448,30 @@ unmsys--file-name
     (setq file (concat (substring file 1 2) ":" (substring file 2))))
   file)
 
+(defun flatten-tree (tree)
+  "Take TREE and \"flatten\" it.
+This always returns a list containing all the terminal nodes, or
+\"leaves\", of TREE.  Dotted pairs are flattened as well, and nil
+elements are removed.
+
+\(flatten-tree \\='(1 (2 . 3) nil (4 5 (6)) 7))
+=> (1 2 3 4 5 6 7)
+
+TREE can be anything that can be made into a list.  For each
+element in TREE, if it is a cons cell return its car
+recursively.  Otherwise return the element."
+    (let (elems)
+    (setq tree (list tree))
+    (while (let ((elem (pop tree)))
+             (cond ((consp elem)
+                    (setq tree (cons (car elem) (cons (cdr elem) tree))))
+                   (elem
+                    (push elem elems)))
+             tree))
+    (nreverse elems)))
+
+;; Technically, `flatten-list' is a misnomer, but we provide it here
+;; for discoverability:
+(defalias 'flatten-list 'flatten-tree)
 
 ;;; subr.el ends here
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index f218a7663e..08f9a697a3 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -372,5 +372,22 @@ subr-test--frames-1
                             (shell-quote-argument "%ca%")))
                    "without-caret %ca%"))))
 
+(ert-deftest subr-tests-flatten-tree ()
+  "Test `flatten-tree' behavior."
+  (should (equal (flatten-tree '(1 (2 . 3) nil (4 5 (6)) 7))
+                 '(1 2 3 4 5 6 7)))
+  (should (equal (flatten-tree '((1 . 2)))
+                 '(1 2)))
+  (should (equal (flatten-tree '(1 nil 2))
+                 '(1 2)))
+  (should (equal (flatten-tree 42)
+                 '(42)))
+  (should (equal (flatten-tree t)
+               '(t)))
+  (should (equal (flatten-tree nil)
+               nil))
+  (should (equal (flatten-tree '(1 ("foo" "bar") 2))
+                 '(1 "foo" "bar" 2))))
+
 (provide 'subr-tests)
 ;;; subr-tests.el ends here
-- 
2.19.1