title: Removing specific rows from a dataframe
body: <p>I have a data frame e.g.:</p>
  
  <pre><code>sub   day
  1      1
  1      2
  1      3
  1      4
  2      1
  2      2
  2      3
  2      4
  3      1
  3      2
  3      3
  3      4
  </code></pre>
  
  <p>and I would like to remove specific rows that can be identified by the combination of sub and day.
  For example say I wanted to remove rows where sub='1' and day='2' and sub=3 and day='4'. How could I do this? 
  I realise that I could specify the row numbers, but this needs to be applied to a huge dataframe which would be tedious to go through and ID each row.</p>