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    It seems that using C in the formula (as in ... + C(Location, Treatment) + ... does the trick, but this results in some pretty ugly category names that I'd like to avoid. Commented Mar 16, 2014 at 13:12
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    I don't understand this. Do you write e.g. C(Location, 'IndianOcean') if you want 'IndianOcean' to be the reference category from the variable 'Location'? Commented Jul 30, 2014 at 12:30