title: How do I create a dataframe from another dataframe with only the last non negative values?
body: <p>I have a multi-indexed dataframe like so:</p>
  <pre><code>          year  value  some_other_column_i_dont_care_about
   one two     
   a   t     2000     0   aaa
       w     2001     3   bbb
       t     2002    -2   ccc
   b   t     2000     4   ddd
       w     2001     7   eee    
       t     2002    -8   fff
   c   t     2000    11   ggg
       w     2001   -12   hhh
       t     2002   -15   iii 
  </code></pre>
  <p>How do I create a new, single level df, that just has the latest (in terms of years) non-negative values, like so:</p>
  <pre><code>          year   value
   one    
   a        2001     3
   b        2001     7
   c        2000    11 
  </code></pre>