title: pandas left join where right is null on multiple columns
body: <p>I have two pandas df x and y, both with the same 3 columns A B C (not nullable). I need to create a new df z, obtained by "subtracting from x the rows which are entirely identical to the rows of y", i.e. a</p>
  
  <pre><code>x left join y on x.A=y.A and x.B=y.B and x.C=y.C
  where y.A is null
  </code></pre>
  
  <p>How would I do that? Got stuck with indexes, concat, merge, join, ...</p>
  
  <p>Example:</p>
  
  <pre><code>dataframe x
  A    B    C
  q1   q2   q3
  q4   q2   q3
  q7   q2   q9
  
  dataframe y
  A    B    C
  q4   q2   q3
  
  dataframe z
  A    B    C
  q1   q2   q3
  q7   q2   q9
  </code></pre>