title: NA == 1 check returns NA
body: <p>Could someone explain me why:</p>
  
  <pre><code>NA == 1 #NA #expect FALSE
  NA == NA #NA #expect TRUE
  1 == NA #NA #expect FALSE
  NA == TRUE #NA #expect FALSE
  </code></pre>
  
  <p>A live example of the usage is <code>df[df$SomeAttribute == 1,]</code> where some of <code>df$SomeAttribute</code> have <code>NA</code> values.  </p>
  
  <p>I know one can use <code>which(df$SomeAttribute == 1)</code> just question the sanity of logic operations on <code>NA</code>.</p>