title: how to know the index of a sublist with only one element
body: <p>The issue is that I want to know the index of a sublist with only one element. For example: </p>
  
  <pre><code>list1=[('CORA', 'As'), ('CORA', '9'), ('DIA', 'K'), ('DIA', 'As'), ('CORA', 'J'), ('CORA', '6'), ('DIA', '7'),]
  </code></pre>
  
  <p>And I want to know the Index of the sublist where "CORA" appears.</p>