Check for org-src--allow-write-back

Not defined in Emacs 25.1. In that case we won't be able to
auto-remove a lang line that we auto-added.

Other small organization changes.

racket-hash-lang.el

@@ -367,7 +367,7 @@ for configuration."
 
        ;; Define org-babel-edit-prep:<lang>.
        (defun ,org-babel-edit-prep-name (_babel-info)
-         (racket--hash-lang-org-babel-edit-prep-for-lang ,lang-str))
+         (racket--hash-lang-org-babel-edit-prep ,lang-str))
 
        (define-derived-mode ,full-mode-name racket-hash-lang-mode
          ,lighter
@@ -396,14 +396,7 @@ for configuration."
       (prog-mode) ;note: resets all buffer-local variables
       (message "hash-lang support not available; needs newer syntax-color-lib"))))
 
-(defun racket--hash-lang-org-babel-edit-prep-for-lang (lang-str)
-  "When the buffer lacks a lang line, add one.
-
-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."
+(defun racket--hash-lang-org-babel-edit-prep (lang-str)
   (racket--hash-lang-maybe-add-lang-line lang-str t))
 
 (defun racket--hash-lang-org-babel-execute (lang-str body params)
@@ -434,14 +427,22 @@ Only supports :result-type output -- not values."
     (org-babel-result-cond result-params result)))
 
 (defun racket--hash-lang-maybe-add-lang-line (lang-str &optional set-write-back-p)
+  "When the buffer lacks a lang line, add one.
+
+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."
   (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
+      (when (and set-write-back-p
+                 (boundp 'org-src--allow-write-back)) ;>25.1
         (setq org-src--allow-write-back
               (lambda ()
                 (when (string= (buffer-substring-no-properties (point-min) end-pos)