title: How can I specify 'unstructured' in lme function?
body: <p>I'm trying to fit a mixed model in R.
  Actually I have a Stata code and want to do the same thing using R.
  The Stata code is</p>
  
  <pre><code>xtmixed laz c.x1##i.j4alloc_n c.x2##i.j4alloc_n ||childuid:age_m, cov(uns) var
  </code></pre>
  
  <p>I can't find how to specify 'unstructured' correlation structure in R.
  In the R help file, there is no 'unstructured'
  <a href="https://stat.ethz.ch/R-manual/R-devel/library/nlme/html/corClasses.html" rel="nofollow">https://stat.ethz.ch/R-manual/R-devel/library/nlme/html/corClasses.html</a></p>