How to limit the possible values taken by an extenal parameter?

 
I would like to limit the choice of possible values taken by an external parameter.
I there a simple way to do than testing the allowed values and displaying an alert if it is different?

For instance, let's say that I have

extern int SL;

and I want that the only authorized values for SL are 10, 20 and 30.

How to do it?

Thanks
 

This feature is planned.

At present insert checking for possible values in the init function

 

After Start(){

////check SL

if ((SL!= 10) ||(SL!=20)||(SL!=30))

{

Alert("SL parameters error!");

return(0);

}