title: Find NA values after using addNA()
body: <p>I have a data frame with a bunch of categorical variables.  Some of them contain NA's and I use the <code>addNA</code> function to convert them to an explicit factor level.  My problem comes when I try to treat them as NA's they don't seem to register.</p>
  
  <p>Here's my example data set and attempts to 'find' NA's:</p>
  
  <pre><code>df1 &lt;- data.frame(id = 1:200, y =rbinom(200, 1, .5),
                    var1 = factor(rep(c('abc','def','ghi','jkl'),50)))
  df1$var2 &lt;- factor(rep(c('ab c','ghi','jkl','def'),50))
  df1$var3 &lt;- factor(rep(c('abc','ghi','nop','xyz'),50))
  
  df1[df1$var1 == 'abc','var1'] &lt;- NA
  
  df1$var1 &lt;- addNA(df1$var1)
  
  df1$isNaCol &lt;- ifelse(df1$var1 == NA, 1, 0);summary(df1$isNaCol)
  df1$isNaCol &lt;- ifelse(is.na(df1$var1), 1, 0);summary(df1$isNaCol)
  df1$isNaCol &lt;- ifelse(df1$var1 == 'NA', 1, 0);summary(df1$isNaCol)
  df1$isNaCol &lt;- ifelse(df1$var1 == '&lt;NA&gt;', 1, 0);summary(df1$isNaCol)
  </code></pre>
  
  <p>Also when I type <code>??addNA</code> I don't get any matches. Is this a gray-market function or something?  Any suggestions would be appreciated.</p>