title: Remove rows which have all NA's, apart from the first column
body: <p>I have a data.table which was formed by taking the differences between two panel observations using:</p>
  <pre><code>tab &lt;- tab[,
     lapply(.SD, function(x) x - shift(x)), 
     by = A, 
     .SDcols = (sapply(tab, is.numeric))
    ]
  
  tab = data.table(A = c(1, 1, 2, 2), B = c(NA, 2, NA, 1), C = c(NA, NA, NA, 2), D=c(NA, 3, NA, 2))
  tab
      A  B  C  D
  1:  1  NA NA NA
  2:  1  2  NA 3
  3:  2  NA NA NA
  4:  2  1  2  2 
  </code></pre>
  <p>I would like to use this <a href="https://stackoverflow.com/questions/38525383/remove-lines-with-only-nas-from-data-table">answer</a>:</p>
  <pre><code>tab &lt;- tab [!Reduce(`&amp;`, lapply(tab , is.na))]
  </code></pre>
  <p>to remove rows 1 and 3, but this does not work because the first column is not NA. How can I adapt the code to solve this?</p>
  <p>Desired outcome:</p>
  <pre><code>    A  B  C  D
  1:  1  2  NA 3
  2:  2  1  2  2 
  </code></pre>