Don't use regexp, DRY, improve docs

Although immediate motivation is byte compiler error in older Emacs
due to rx not supporting "literal" clause, it is also the case that we
need not use a regexp at all.

racket-hash-lang.el

@@ -313,18 +313,19 @@ named `racket-hash-lang:LANG-mode'.
 LANG should be an unquoted symbol, same as you would use in a
 Racket #lang line.
 
-EXT should be a string with the file extension for LANG, not
+EXT should be a string with the file extension for LANG, /not/
 including any dot.
 
 For example (racket-define-hash-lang rhombus \"rhm\")
 
-- defines `racket-hash-lang:rhombus-mode'.
+1. Defines `racket-hash-lang:rhombus-mode'.
 
-- configures things like `auto-mode-alist', `org-src-lang-modes',
-  and `org-babel-tangle-lang-exts'.
+2. Configures things like `auto-mode-alist', `org-src-lang-modes',
+   and `org-babel-tangle-lang-exts'.
 
-- allows a buffer to omit the explicit #lang line, when it is by
-  `org-mode'.
+3. Allows a buffer to omit the explicit #lang line, when it is
+   created by `org-mode' for user editing or formatting of a
+   source code block whose language property is \"rhombus\".
 
 Although `racket-hash-lang-mode' works for any Racket hash-lang
 simply by starting the buffer with a #lang line, some features in
@@ -403,17 +404,7 @@ Otherwise things like `racket-xp-mode' will report errors.
 IFF we add one, arrange for a write-back function to remove it.
 Note: `org-src--contents-for-write-back' strips text properties
 so we can't insert a propertized string to look for later."
-  (save-excursion
-    (goto-char (point-min))
-    (unless (looking-at-p (rx bol "#lang" (1+ " ") (literal lang-str) "\n"))
-      (let ((lang-line-str (concat "#lang " lang-str "\n")))
-        (insert lang-line-str)
-        (setq org-src--allow-write-back
-              (lambda ()
-                (save-excursion
-                  (goto-char (point-min))
-                  (when (looking-at-p (rx bol (literal lang-line-str)))
-                    (delete-region (point-min) (1+ (length lang-line-str)))))))))))
+  (racket--hash-lang-maybe-add-lang-line lang-str t))
 
 (defun racket--hash-lang-org-babel-execute (lang-str body params)
   "A basic way to run Racket programs using any #lang.
@@ -435,17 +426,28 @@ Only supports :result-type output -- not values."
                  body))
          (tmp-src-file (org-babel-temp-file "racket-hash-lang-src-" ".rkt"))
          (_ (with-temp-file tmp-src-file
-              ;; Unless there's already a #lang line (according to
-              ;; <100% reliable regexp test) add one.
-              (unless (string-match-p (rx bol "#lang" (1+ " ") (literal lang-str) "\n")
-                                      body)
-                (insert "#lang " lang-str "\n"))
-              (insert body)))
+              (insert body)
+              (racket--hash-lang-maybe-add-lang-line lang-str nil)))
          (cmdline (concat racket-program " " tmp-src-file))
          (result (org-babel-eval cmdline "")))
     (delete-file tmp-src-file)
     (org-babel-result-cond result-params result)))
 
+(defun racket--hash-lang-maybe-add-lang-line (lang-str &optional set-write-back-p)
+  (let* ((lang-line-str (concat "#lang " lang-str "\n"))
+         (end-pos (1+ (length lang-line-str))))
+    (unless (string= (buffer-substring-no-properties (point-min) end-pos)
+                     lang-line-str)
+      (save-excursion
+        (goto-char (point-min))
+        (insert lang-line-str))
+      (when set-write-back-p
+        (setq org-src--allow-write-back
+              (lambda ()
+                (when (string= (buffer-substring-no-properties (point-min) end-pos)
+                               lang-line-str)
+                  (delete-region (point-min) end-pos))))))))
+
 ;; org-babel support
 ;;
 ;; The above suffices for edit and tangle. Suffices for execute in