title: How to quickly replicate/update local library under $R_LIBS_USER?
body: <p>Suppose that</p>
  
  <ol>
  <li>the version of R I have installed is 3.3.1;</li>
  <li>my environment variable <code>$R_LIBS_USER</code> is set to <code>$HOME/my/R_lib/%V</code>; and</li>
  <li>I have a directory <code>$HOME/my/R_lib/3.3.1</code> containing a large number of packages I've installed over time.</li>
  </ol>
  
  <p>Now I want to upgrade my version of R, to 3.4.1, say.</p>
  
  <p>I'm looking for a convenient way to install a new collection of packages under the new directory <code>$HOME/my/R_lib/3.4.1</code> that is the "version-3.4.1-equivalent" of the library I currently have under <code>$HOME/my/R_lib/3.3.1</code>.</p>
  
  <p>(IOW, I'm looking for a functionality similar to what one can do with Python's <code>pip</code> installer's "freeze" option, which, essentially, produces the input one would have to give the installer in the future to reproduce the current installation.)</p>