[Groff] Conditional problem: "!" operator after "&"

[Larry Kollar]

I'm seeing this in both the release and this morning's CVS versions. A test 
file that illustrates the issue:

mac-lkollar:/tmp lkollar$ cat -n test.t
     1  .nr NCS 1
     2  .nr SIP 1
     3  .LP
     4  This is a test,
     5  .if \n[NCS]&!\n[SIP] .nop NCS only.
     6  .if !\n[NCS]&\n[SIP] .nop SIP only.
     7  .if \n[NCS]&\n[SIP] .nop NCS and SIP.
     8  .if !\n[NCS]&!\n[SIP] .nop actually it isn't because nothing is defined!
mac-lkollar:/tmp lkollar$

Running groff on this file produces the warnings:

test.t:5: warning: numeric expression expected (got `!')
test.t:8: warning: numeric expression expected (got `!')

Using parentheses doesn't change the results at all. Can anyone suggest a fix 
or at least a workaround?

Thanks,
-- Larry