title: How can I define a ScalaCheck generator that produces a subset of a sequence's elements?
body: <p>How can I pick a subset of the elements of a sequence?</p>
  
  <p>For instance, if I had the sequence <code>Seq(1,2,3,4,5)</code>, I'd like each call to my generator to produce something like</p>
  
  <pre><code>Seq(1,4)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>Seq(1,2,3,5)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>Seq()
  </code></pre>
  
  <p>How can I define such a generator?</p>