BIRD 2.0 Syntax issue with defined()

[Elmar K. Bins]

Hello fellows,

I have my bird config failing on something as trivial as

if defined (some_variable_never_defined)
  then ... ;

with "syntax error, unexpected CF_SYM_UNDEFINED" on the "some_...".

Am I wrongly assuming that defined() would check for the variable to exist
within BIRD's global scope (e.g., appear in the namespace)?

How, if not this way, can I check for a variable to have been defined prior
(and, if it does, do certain things, of course)?

Elmar.

PS: The first example on the filters page makes me believe that defined() works
    in exactly this way, but I might be mistaken.