title: "Anti-merge" in pandas (Python)
body: <p>How can I pick out the difference between to columns of the same name in two dataframes?
  I mean I have dataframe A with a column named X and dataframe B with column named X, if i do <code>pd.merge(A, B, on=['X'])</code>, i'll get the common X values of A and B, but how can i get the "non-common" ones?</p>