Compound variables

To avoid mysterious errors you should always use some special chars as 
first chars of a tail (for example two underscores __) and never use these 
chars for normal variables (or vice versa). 

Another nice method to avoid errors: Always use a number (0..9) for the 
first character of the tails. This method works because variable names 
can't begin with a number.   

Source:  I found this hint in the documentation for the DLL RXU