title: How can I manipulate strings in a slice of a pandas MultiIndex
body: <p>I have a <code>MultiIndex</code> like this:</p>
  
  <pre><code>                                 metric
  sensor  variable   side  
  foo        Speed   Left      Left speed
                    Right     Right speed
  bar        Speed   Left      Left_Speed
                    Right     Right_Speed
  baz        Speed   Left           speed
  foo      Support   Left    Left support
                    Right   Right support
  bar      Support   Left    Left_support
                    Right   Right_support
  baz      Support   Left         support
  </code></pre>
  
  <p>I'm trying to apply a string mapping to a slice of this dataframe:</p>
  
  <pre><code>df.loc['baz',:,'Left'].metric.map(lambda s: "Left_" + s)
  </code></pre>
  
  <p>How can I apply this map to just the <code>baz-Left</code> rows, and get back the resulting <code>DataFrame</code>?</p>
  
  <pre><code>                                 metric
  sensor  variable   side  
  foo        Speed   Left      Left speed
                    Right     Right speed
  bar        Speed   Left      Left_Speed
                    Right     Right_Speed
  baz        Speed   Left      Left_speed
  foo      Support   Left    Left support
                    Right   Right support
  bar      Support   Left    Left_support
                    Right   Right_support
  baz      Support   Left    Left_support
  </code></pre>