[BioC] DESeq - working with more than 2 conditions

[Shanrong Zhao]

Section 4 in DESeq tutorial covers multiple-factor, but multi-factor is NOT 
equal to multiple conditions.

I believe in real world, more often you have more conditions than just two. 
Let's say I have a study:
1. 3 conditions: control, treat_1, and treat_2
2. Triplicates in each condition
3. comparisons:  
    a:  treat_1 vs control
    b:  treat_2 vs control

Questions: how to do it using DESeq?  I did google around, search 
Seqanswrs ..., but still wondering.

Thanks,
Shanrong