title: Check for missing number in sequence
body: <p>I have an <code>List&lt;int&gt;</code> which contains 1,2,4,7,9 for example.</p>
  
  <p>I have a range from 0 to 10.</p>
  
  <p>Is there a way to determine what numbers are missing in that sequence?</p>
  
  <p>I thought LINQ might provide an option but I can't see one</p>
  
  <p>In the real world my List could contain 100,000 items so performance is key</p>