title: How to remove a pandas dataframe from another dataframe
body: <p>How to remove a pandas dataframe from another dataframe, just like the set subtraction:</p>
  
  <pre><code>a=[1,2,3,4,5]
  b=[1,5]
  a-b=[2,3,4]
  </code></pre>
  
  <p>And now we have two pandas dataframe, how to remove df2 from df1:</p>
  
  <pre><code>In [5]: df1=pd.DataFrame([[1,2],[3,4],[5,6]],columns=['a','b'])
  In [6]: df1
  Out[6]:
     a  b
  0  1  2
  1  3  4
  2  5  6
  
  
  In [9]: df2=pd.DataFrame([[1,2],[5,6]],columns=['a','b'])
  In [10]: df2
  Out[10]:
     a  b
  0  1  2
  1  5  6
  </code></pre>
  
  <p>Then we expect df1-df2 result will be:</p>
  
  <pre><code>In [14]: df
  Out[14]:
     a  b
  0  3  4
  </code></pre>
  
  <p>How to do it? </p>
  
  <p>Thank you.</p>