title: Factor with reference level in R?
body: <p>In R there are factors and there are ordered factors. Is there anything "in between", so to speak? factors are completely unordered, while ordered factors are completely ordered; I want a factor type that has a reference level. This would be useful in tagging a control group, for instance, that I always want to consider as a "baseline".</p>
  
  <p>I have seen <code>relevel()</code> in R, but this is not what I want, since there is no way (apparently) to tell the difference between a factor whose reference level is the first level, and a regular factor.</p>
  
  <p>Does anyone know how to tag a reference level in R, or if not, how I can extend the factor class to do so?</p>