code shortcut to trigger export (with <Ctrl-E>); it outputs syntatically valid .sml, but I am not sure if the output is correct as intended so it needs testing

imperative-shell/file-thread.sml

@@ -143,7 +143,7 @@ struct
         case Mailbox.recv fileMailbox of
           SAVE_TRIANGLES triangles => saveTriangles triangles
         | LOAD_TRIANGLES => loadTriangles inputMailbox
-        | EXPORT_TRIANGLES triangles => ()
+        | EXPORT_TRIANGLES triangles => exportTriangles triangles
     in
       run (fileMailbox, inputMailbox)
     end