er, and instead worked through several false starts (first not
realising that it was a common problem, then with the
<code>_each()</code> functions, and most recently with the
<code>_if()</code>/<code>_at()</code>/<code>_all()</code> functions).
But <code>across()</code> couldn’t work without three recent
discoveries:</p>
<ul>
<li><p>You can have a column of a data frame that is itself a data
frame. This is something provided by base R, but it’s not very well
documented, and it took a while to see that 