title: How to replace values by NA in stack?
body: <p>Hello I am giving here a reproduc example. I have three rasters and I stacked them.
  I want to replace any value above 50 by NA but I am getting an error:</p>
  
  <pre><code>     r &lt;- raster(nrows=10, ncols=10)
       r &lt;- setValues(r, 1:ncell(r))
       r1 &lt;- raster(nrows=10, ncols=10)
       r1 &lt;- setValues(r1, 1:ncell(r))
       r2 &lt;- raster(nrows=10, ncols=10)
       r2 &lt;- setValues(r2, 1:ncell(r))
       St=stack(r,r1,r2)
  
       NAvalue(St) &lt;- &gt;50
     Error: unexpected '&gt;=' in "NAvalue(St) =  &gt;="
  </code></pre>
  
  <p>Any help?Thanks</p>