title: How to mutate several columns by column index rather than column name using across?
body: <p>Building on this questions:</p>
  <p><a href="https://stackoverflow.com/questions/43137868/r-dplyr-mutate-on-column-index">R dplyr mutate on column index</a></p>
  <p><a href="https://stackoverflow.com/questions/32618744/dplyr-how-to-reference-columns-by-column-index-rather-than-column-name-using-mu">dplyr: how to reference columns by column index rather than column name using mutate?</a></p>
  <p>I want to mutate several columns using column indexes for both the source and the destination of the mutate like:</p>
  <pre><code>df &lt;- tribble(~C1,~C2,~C3,1,2,3,4,5,6,7,8,9)
  dfn &lt;- df
  dfn[,1:2] = dfn[,3]   # like this in dplyr
  
  df &lt;- df %&gt;% mutate(accross(.[[1:2]]) = .[[3]])
  </code></pre>
  <p>gives:</p>
  <pre><code>Fehler: Unerwartete(s) '=' in &quot;df &lt;- df %&gt;% mutate(accross(.[[1:2]]) =&quot;
  Error: Unexpected '=' in &quot;df &lt;- df %&gt;% mutate(accross(.[[1:2]]) =&quot;
  </code></pre>