title: Add a prefix to column names
body: <p>When reading the following <a href="http://stat.ethz.ch/R-manual/R-patched/library/base/html/colnames.html">helpfile</a> it should be possible to add a prefix to the column names :</p>
  
  <pre><code>colnames(x, do.NULL = TRUE, prefix = "col")
  </code></pre>
  
  <p>The following doesn't work for me. What am I doing wrong here?</p>
  
  <pre><code>m2 &lt;- cbind(1,1:4)
  colnames(m2, do.NULL = FALSE)
  colnames(m2) &lt;- c("x","Y")
  colnames(m2) &lt;- colnames(m2, prefix = "Sub_")
  colnames(m2)
  </code></pre>