done coding functionality to create a string suitable for exporting

fcore/common-update.sml

@@ -10,7 +10,7 @@ struct
   fun getSaveSquaresMsg (model: app_type) =
     let
       val {canvasWidth, canvasHeight, squares, ...} = model
-      val str = CollisionTree.toString (squares, canvasWidth, canvasHeight)
+      val str = CollisionTree.toSaveString (squares, canvasWidth, canvasHeight)
       val msg = SAVE_SQUARES str
     in
       (model, [FILE msg])
@@ -19,9 +19,17 @@ struct
   fun getLoadSquaresMsg model =
     (model, [FILE LOAD_SQUARES])
 
-  (* unimplemented *)
-  fun getExportSquaresMsg model = (model, [])
+  fun getExportSquaresMsg (model: app_type) =
+    let
+      val {squares, canvasWidth, canvasHeight, ...} = model
+      val exportString =
+        CollisionTree.toExportString (squares, canvasWidth, canvasHeight)
+      val msg = EXPORT_SQUARES exportString
+    in
+      (model, [FILE msg])
+    end
 
+  (* unimplemented *)
   fun useSquaresInNormalMode (model, squares) = (model, [])
 
   fun squaresLoadError model = (model, [])