Enforce Scala Seq to use a single type only (no LUB)
<p>I would like to create and use a <code>Seq[T]</code> collection in Scala
and ensure that it only use one type. So if I use:</p>

<pre><code>val l = List(1, 2, 2.0)
</code></pre>

<p>a compile time error should occur - <code>List</code> elements should all be 
<code>Double</code> or all <code>Int</code>.</p>