title: Kotlin sequence concatenation
body: <pre><code>val seq1 = sequenceOf(1, 2, 3)
  val seq2 = sequenceOf(5, 6, 7)
  sequenceOf(seq1, seq2).flatten().forEach { ... }
  </code></pre>
  
  <p>That's how I'm doing sequence concatenation but I'm worrying that it's actually copying elements, whereas all I need is an iterator that uses elements from the iterables (seq1, seq2) I gave it.</p>
  
  <p>Is there such a function?</p>