title: Filtering multiple items in a multi-index Python Panda dataframe
body: <p>I have the following table:</p>
  <pre><code>NSRCODE  PBL_AWI          Area           
  CM       BONS             44705.492941
           BTNN            253854.591990
           FONG             41625.590370
           FONS             16814.159680
           Lake             57124.819333
           River             1603.906642
           SONS            583958.444751
           STNN             45603.837177
           clearcut        106139.013930
           disturbed       127719.865675
           lowland         118795.578059
           upland         2701289.270193
  LBH      BFNN            289207.169650
           BONS           9140084.716743
           BTNI             33713.160390
           BTNN          19748004.789040
           FONG           1687122.469691
           FONS           5169959.591270
           FTNI            317251.976160
           FTNN           6536472.869395
           Lake            258046.508310
           River            44262.807900
           SONS           4379097.677405
           burn regen      744773.210860
           clearcut         54066.756790
           disturbed       597561.471686
           lowland       12591619.141842
           upland        23843453.638117
  </code></pre>
  <p>Note: Both <code>NSRCODE</code> and <code>PBL_AWI</code> are indices.</p>
  <p>How do I search for values in column <code>PBL_AWI</code>? For example I want to keep the values <code>['Lake', 'River', 'Upland']</code>.</p>