built-in variables can be used in outer scopes.

src/main/java/carpet/script/value/GlobalValue.java

@@ -1,10 +1,13 @@
 package carpet.script.value;
 
+import carpet.script.exception.InternalExpressionException;
+
 public class GlobalValue extends Value
 {
     public GlobalValue(Value variable)
     {
-        variable.assertAssignable();
+        if (variable.boundVariable == null)
+            throw new InternalExpressionException("You can only borrow variables from the outer scope");
         this.boundVariable = variable.boundVariable;
     }