title: Delete Variable only if ALWAYS 0 in Panel Data?
body: <p>I have a panel data frame in which the number of children a woman has is asked. Now I would like to delete all women who <em>DON´T</em> have children, while maintaining women who f.e. didn´t have a child in 2016, but in 2018.
  Here´s part of the data frame for reference:</p>
  <pre><code>ID  year    child
  1   2012    0
  1   2014    0
  1   2016    1
  2   2012    0
  2   2014    0
  2   2016    0
  3   2014    1
  3   2016    1
  4   2012    0
  4   2016    1
  4   2018    2
  5   2018    0
  5   2020    0
  </code></pre>
  <p>Can someone help me delete all women who are not mothers?</p>