dplyr: transpose a list (length-n) in a cell to n-rows : <p>I have this style of data:</p>

<pre><code>df=tibble(a=letters[23:25],b='k',c='m',d=list(runif(4)))
df
# A tibble: 3 x 4
      a     b     c         d
  &lt;chr&gt; &lt;chr&gt; &lt;chr&gt;    &lt;list&gt;
1     w     k     m &lt;dbl [4]&gt;
2     x     k     m &lt;dbl [4]&gt;
3     y     k     m &lt;dbl [4]&gt;
</code></pre>

<p>I'd like to have each value in the list <code>d</code> be an entry in a column named <code>m</code>, which is the contents of <code>c</code>.  Specifically:</p>

<pre><code># A tibble: 3 x 12
      a     b     m     
  &lt;chr&gt; &lt;chr&gt; &lt;dbl&gt;     
1     w     k     0.57 
2     x     k     0.45 
3     y     k     0.34 
</code></pre>

<p>If possible I'd like to use only <code>dplyr</code>, or at least something from the <code>tidyverse</code></p>
