[R] interactions in repeated measures ANOVA

[Nicola Spotorno]

Hello all,
I apply Repeated measures ANOVA for analyzing ERPs  data.  To do this I 
use lme() function and glht() for the post hoc analysis.
I followed the very useful suggestions found in this help list but I 
have a problem. I do not know how to test the interaction between 
different levels of my predictors.
Consider I have 2 predictors, the first one has three levels: A,B,C; 
also the second one has three levels:1,2,3. How can I investigate, for 
example, the interaction between A and 2?

Thank you in advance for your help.

Nicola