title: How to check whether a list contains `NA`?
body: <p>When the right hand side is a vector, <code>%in%</code> can be used to check for <code>NA</code>s:</p>
  
  <pre><code>&gt; NA %in% c(NA, 2)
  [1] TRUE
  &gt; NA %in% c(1, 2)
  [1] FALSE
  &gt; 1 %in% c(NA, 2)
  [1] FALSE
  &gt; 1 %in% c(1, 2)
  [1] TRUE
  </code></pre>
  
  <p>When the right hand side is a list, <code>%in%</code> behaves differently:</p>
  
  <pre><code>&gt; NA %in% list(NA, 2)
  [1] FALSE
  &gt; NA %in% list(1, 2)
  [1] FALSE
  &gt; 1 %in% list(NA, 2)
  [1] FALSE
  &gt; 1 %in% list(1, 2)
  [1] TRUE
  </code></pre>
  
  <p>Is this a bug or a feature?  Is this described in the documentation?</p>