C-M-x evaluates a top-level form, where every other Lisp puts it

Emacs binds it in emacs-lisp-mode-map and nowhere else, so a mode derived
from lisp-mode inherits nothing and the key answers "undefined" — which
reads as the client being broken rather than as the key being free.

emacs/MANUAL.md

@@ -743,6 +743,7 @@ Use `C-c C-g` if you need frames.
 |---|---|
 | `C-c C-c` | the top-level form at point, recompiled and installed |
 | `C-u C-c C-c` | ...and stop at the form point is inside (`C-u C-u`: on entry) |
+| `C-M-x` | the same as `C-c C-c`, on the binding SLIME and CIDER use |
 | `C-c C-k` | the whole buffer, as one module |
 | `C-x C-e` | the expression before point, evaluated in the running program |
 | `C-u C-x C-e` | ...and stop at it instead of printing its value |

emacs/flan-mode.el

@@ -198,6 +198,11 @@ line is off screen."
   (let ((map (make-sparse-keymap)))
     ;; Autoloaded from flan.el, so the client loads on first use.
     (define-key map (kbd "C-c C-c") #'flan-eval-defun)
+    ;; The same command on the binding SLIME and CIDER put it on.  Emacs binds
+    ;; C-M-x to eval-defun only in `emacs-lisp-mode-map', so a mode derived
+    ;; from `lisp-mode' inherits nothing and the key is undefined — which
+    ;; reads as the client being broken rather than as the key being free.
+    (define-key map (kbd "C-M-x") #'flan-eval-defun)
     (define-key map (kbd "C-c C-k") #'flan-eval-buffer)
     (define-key map (kbd "C-x C-e") #'flan-eval-last-sexp)
     (define-key map (kbd "C-c C-z") #'flan-connect)

sand.flan

@@ -180,6 +180,9 @@
                              (rl/get-color c))))))
   (rl/draw-fps 20 20))
 
+(defvar frame Allocator (arena-new 262144))
+(defvar game-data (embed (with-allocator frame )))
+
 (defn main [] ()
   (rl/set-trace-log-level :warning)
   (rl/init-window screen-width screen-height "SAND")