title: R: Find vector elements containing multiple string matches
body: <p>I would like to find the elements of a vector (strings) which contain all of the strings specified by another vector. For example,</p>
  
  <pre><code>x &lt;- c("xxxabcxdxexfxx", "xxaxbcdexx", "xaxxxbc")
  a &lt;- c("a", "b", "c", "d", "e", "f")
  </code></pre>
  
  <p>I would like to find the elements of <code>x</code> that contain all the strings in <code>a</code>, so to get</p>
  
  <pre><code>[1] TRUE FALSE FALSE
  </code></pre>