title: Check if an element is within a sequence
body: <p>how to check if an element is contained within a sequence? I expected some Seq.contains, but i could not find it. Thanks</p>
  
  <p><strong>EDIT</strong>:
  Or, for an easier task, how to make the diff between two sequences? Like, getting all the elements within a list that doesn not belong to another (or that do)?</p>