[Xml-compile] NIL vs. undef?

[E R]

I've read the docs, but I still have some questions about the use of NIL:

1. How do you disambiguate the NIL value from a string value of 'NIL'
(the three letter string)?

2. When I get an $answer back from a SOAP call, I'd like to translate
all NIL values to undef. Is there a standard way to do this?

Thanks,
ER