s returned
by <code>f</code> for each element of this, in iteration order.</p>
<p>The returned <a href="dart-core/Iterable-class.html">Iterable</a> is lazy, and calls <code>f</code> for each element
of this every time it's iterated.</p>
<p>Example:</p>
<pre class="language-dart"><code>var pairs = [[1, 2], [3, 4]];
var flattened = pairs.expand((pair) =&gt; pair).toList();
print(flattened); // =&gt; [1, 2, 3, 4];

var input = [1, 2, 3];
var duplicated = input.expand((i) =&gt; [i, i]).toList();
print(duplicate