title: Harnessing .f list names with purrr::pmap
body: <p>The following works ok:</p>
  
  <pre><code>pmap_dbl(iris, ~ ..1 + ..2 + ..3 + ..4)
  </code></pre>
  
  <p>The documentation for <code>.l</code> provides for <code>A list of lists. ... List names will be used if present.</code>.  This suggests you should be able to work with the list names (i.e. column names). However:</p>
  
  <pre><code>pmap_dbl(iris, ~ Sepal.Length + Sepal.Width + Petal.Length + Petal.Width)
  Error in .f(Sepal.Length = .l[[c(1L, i)]], Sepal.Width = .l[[c(2L, i)]],  : 
    object 'Sepal.Length' not found
  </code></pre>
  
  <p>How are list names harnessed in practice?</p>