R help - Jan 2008 - how to use anova() to test the sum of coefficients in nlme package

Dear R-help,

In nlme package, anova () can be used to test the difference between
two coefficients as shown on page 225
of "Mixed Effects Models in S and S-Plus":

anova(fm2BW.lme, L = c("Time:Diet2" = 1, "Time:Diet3" = -1))

Now my question is instead of test the difference between two
coefficients, can I use anova to test the significance of the sum of
two coefficients, like

anova(fm2BW.lme, L = c("Time:Diet2" = 1, "Time:Diet3" = 1))

Thanks,

Shirley