[R] Constrain coefs. in linear model to sum to 0

[Gorjanc Gregor]

Hello!

I would like to use constrain to sum coeficients of a factor to 0 instead
of classical corner contraint i.e. I would like to fit a model like

lm(y ~ 1 + effectA + effectB)

and say get parameters

intercept
effectA_1
effectA_2
effectB_1
effectB_2
effectB_3

where effectA_1 represents deviation of level A_1 from intercept and 
sum(effectA_1, effectA_2) = 0 and the same for factor B.

Is this possible to do?

Lep pozdrav / With regards,
    Gregor Gorjanc

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University of Ljubljana     PhD student
Biotechnical Faculty        URI: http://www.bfro.uni-lj.si/MR/ggorjan
Zootechnical Department     mail: gregor.gorjanc <at> bfro.uni-lj.si
Groblje 3                   tel: +386 (0)1 72 17 861
SI-1230 Domzale             fax: +386 (0)1 72 17 888
Slovenia, Europe
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"One must learn by doing the thing; for though you think you know it,
 you have no certainty until you try." Sophocles ~ 450 B.C.