[R] contrasts in aov

[John Vokey]

useRs,
   A no doubt simple question, but I am baffled.  Indeed, I think I  
once knew the answer, but can't recover it.  The default contrasts  
for aov (and lm, and...) are contr.treatment and contr.poly for  
unordered and ordered factors, respectively.  But, how does one  
invoke the latter?  That is, in a data.frame, how does one indicate  
that a factor is an *ordered* factor such that contr.poly is invoked  
in the aov or lm call?
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-Dr. John R. Vokey