title: fct_recode replace level by NA
body: <p>I am trying to substitute all &quot;a&quot; in a R factor variable by a NA character using forcats::fct_recode. This is what i tried:</p>
  <pre><code>fct &lt;- forcats::as_factor(c(&quot;a&quot;, &quot;b&quot;))
  fct %&gt;% forcats::fct_recode(&quot;c&quot; = &quot;a&quot;) #works
  fct %&gt;% forcats::fct_recode(NA = &quot;a&quot;) #error
  fct %&gt;% forcats::fct_recode(NA_character_ = &quot;a&quot;) #error
  </code></pre>
  <p>Is there a way to achieve my goal with fct_recode?</p>