title: Drop all rows before first occurrence of a value
body: <p>I have a df like so:</p>
  
  <pre><code>Year  ID Count
  1997  1  0
  1998  2  0
  1999  3  1
  2000  4  0
  2001  5  1
  </code></pre>
  
  <p>and I want to remove all rows before the first occurrence of <code>1</code> in <code>Count</code> which would give me:</p>
  
  <pre><code>Year  ID Count
  1999  3  1
  2000  4  0
  2001  5  1
  </code></pre>
  
  <p>I can remove all rows AFTER the first occurrence like this:</p>
  
  <pre><code>df=df.loc[: df[(df['Count'] == 1)].index[0], :]
  </code></pre>
  
  <p>but I can't seem to follow the slicing logic to make it do the opposite.</p>