Create a new column on a pandas pivot_table with multiindex
<p>This seems so simple I can't believe I'm pulling my hairs.
I have a pivot_table like this</p>

<pre>
Name      income          expenses
         2015  2016       2015  2016
Joe Doe     2     4          5     7
Jane Doe    2     4          5     7
Doe Joe     2     4          5     7
Doe Jane    2     4          5     7
</pre>

<p>I simply want to add a calculated column  profit_loss = (income - expenses)
I thought it would be something like:</p>

<pre><code> df['profit_loss'] = df['income'] - df['expenses]
</code></pre>

<p>I only get errors.  </p>

<p>Without having to write a lot of code or preparation to the base table that creates this pivot_table, is there an easier way to deal with <code>MultiIndexes</code> on a pandas pivot_table ?</p>