In trunk, see:
* SOLR-2335: New 'field(...)' function syntax for refering to complex
field names (containing whitespace or special characters) in functions.
The schema in trunk also specifies:
!-- field names should consist of alphanumeric or underscore
characters only and
not start
I know there are edge cases where odd field naming causes
problems, field names not well-defined/enforced with Solr. Rather than
banging my head against the wall and finding these cases
at inopportune moments, I'd confine myself to lower-case
and underscores.
Other stuff _may_ work, like capital
