===== What does "compiling" the overlay do? =====
In order to be able to run, code must be compiled.  To test whether your overlays will compile, use the overlay tool in the scenario window.  A green check (**a**) will indicate successful compilation.  A red **X** indicates a compile failure.  

The compiler (and the compile check in the overlay tool) checks the syntax of the code in the overlay, and that the instance variables and arguments used exist (i.e. are defined in code).  (Note:  Instance variables and arguments __can__ be defined in class overlays, but the overlay using ("sending") them will remain a red X).  Compiling does __not__ check to confirm if the methods used in the overlay actually exist in the baseline code, since the user could define them in another overlay.
