title: gls() vs. lme() in the nlme package
body: <p>In the nlme package there are two functions for fitting linear models (lme and gls). </p>
  
  <ol>
  <li>What are the differences between
  them in terms of the types of models
  that can be fit, and the fitting
  process?  </li>
  <li>What is the design
  rational for having two functions to
  fit linear mixed models where most
  other systems (e.g. SAS SPSS) only
  have one?</li>
  </ol>
  
  <p>Update: Added bounty. Interested to know differences in the fitting process, and the rational.</p>