title: Double nesting in the tidyverse
body: <p>Using the <a href="http://r4ds.had.co.nz/many-models.html" rel="noreferrer">examples</a> from Wickhams introduction to purrr in R for data science, I am trying to create a double nested list.</p>
  
  <pre><code>library(gapminder)
  library(purrr)
  library(tidyr)
  gapminder
  nest_data &lt;- gapminder %&gt;% group_by(continent) %&gt;% nest(.key = by_continent) 
  </code></pre>
  
  <p>How can I further nest the countries so that nest_data contains by_continent and a new level of nesting by_contry that ultimately includes the tibble by_year?</p>
  
  <p>Furthermore, after creating this datastructure for the gapminder data - how would you run the regression model examples from the <a href="http://r4ds.had.co.nz/many-models.html" rel="noreferrer">bookchapter</a> for each country?</p>