title: Delete lines in a dataframe based on lookup in another dataframe
body: <p>I work with two dataframes. I want to remove lines in a first dataframe based on a match in another one.</p>
  
  <p>In df1 I have two columns (called Type1 &amp; Type2) + a flag.
  I want to delete lines where flag = True &amp; where Type1 &amp; Type2 match a combination in another df2.</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df1 = pd.DataFrame(np.random.randint(0,10,size=(100, 2)),columns = ["Type1","Type2"])
  df1["Flag"] = np.random.randint(0,10,size=(100))&gt;6
  df1.head()
  
    Type1  Type2   Flag
  0      8      5  False
  1      1      6  False
  2      9      2  False
  3      0      9   True
  4      2      9  False
  
  df2 = pd.DataFrame(np.random.randint(0,10,size=(100, 2)),columns = ["Type1","Type2"])
  df2.head()
  
    Type1  Type2
  0      0      9
  1      7      8
  2      5      1
  3      3      3
  4      3      2
  </code></pre>
  
  <p>For example here the line in df1 where index=3 should be deleted as Flag=True and (0,9) exists in df2.</p>