[R] Linear model - coefficients

[Robert Ruser]

Dear R Users,
Using lm() function with categorical variable R use contrasts. Let
assume that I have one X independent variable with 3-levels. Because R
estimate only 2 parameters ( e.g. a1, a2)  the coef function returns
only 2 estimators. Is there any function or trick to get another a3
values. I know that using contrast sum (?contr.sum) I could compute a3
= -(a1+a2). But I have many independent categorical variables and I'm
looking for a fast solution.

Robert