title: Applying transformation to every item in Single<List<T>>
body: <p>I have a <code>val singleList: Single&lt;List&lt;EntityItem&gt;&gt; = ...</code></p>
  
  <p>I also have a custom transformer to convert <code>ItemEntity</code> to <code>Item</code>
  (doing either <code>transformer::fromEntity</code> or <code>transformer.fromEntity(entity)</code>)</p>
  
  <p>How would I use RxJava operators to run this transformation on each <code>EntityItem</code> in <code>singleList</code> so that <code>Single&lt;List&lt;EntityItem&gt;&gt;</code> becomes <code>Single&lt;List&lt;Item&gt;&gt;</code></p>