title: Can dplyr join on multiple columns or composite key?
body: <p>I realize that <code>dplyr</code> v3.0 allows you to join on different variables:</p>
  
  <p><code>left_join(x, y, by = c("a" = "b")</code> will match <code>x.a</code> to <code>y.b</code></p>
  
  <p>However, is it possible to join on a combination of variables or do I have to add a composite key beforehand?</p>
  
  <p>Something like this:</p>
  
  <p><code>left_join(x, y, by = c("a c" = "b d")</code> to match the concatenation of [<code>x.a</code> and <code>x.c</code>] to [<code>y.b</code> and <code>y.d</code>]</p>