title: Use na.rm=TRUE with functions inside mutate
body: <p>I am trying to calculate cumulative sums and am using <code>mutate</code> to create the new column.  I am doing this for multiple columns and each has missing data in different places.</p>
  
  <pre><code>day    water    nitrogen
   1      4        5
   2      NA       6
   3      3        NA
   4      7        NA
   5      2        9
   6      NA       3
   7      2        NA
   8      NA       2
   9      7        NA
  10      4        3
  </code></pre>
  
  <p>I try </p>
  
  <pre><code>mutate(df, sumwater = cumsum(water))%&gt;%
  mutate(sumnitrogen = cumsum(nitrogen)) 
  </code></pre>
  
  <p>or</p>
  
  <pre><code>mutate(df, sumwater = cumsum(water, na.rm = TRUE))%&gt;%
  mutate(sumnitrogen = cumsum(nitrogen, na.rm = TRUE))
  </code></pre>
  
  <p>neither works.  I know we can do <code>na.rm=TRUE</code> in <code>summarize</code> functions.  Is there a way to get it to work in <code>mutate</code>? I prefer a <code>dplyr</code> answer because in reality this part of a long chain of piping.</p>