title: R dimnames of matrix, strange behaviour
body: <p>Is this a bug in R or does it make sense? </p>
  
  <pre><code>## works
  aa &lt;- matrix(nrow=1,ncol=2)
  dimnames(aa)[[2]] &lt;- c("a","b")
  dimnames(aa)[[1]] &lt;- c("c")
  
  ## does not work
  bb &lt;- matrix(nrow=1,ncol=2)
  dimnames(bb)[[1]] &lt;- c("c")
  Error in dimnames(bb)[[1]] &lt;- c("c") : 'dimnames' must be a list
  </code></pre>
  
  <p>Thanks for explanations !</p>