<p>I would like to be able to fusion an <code>IEnumerable&lt;IEnumerable&lt;T&gt;&gt;</code> into <code>IEnumerable&lt;T&gt;</code> (i.e. merge all individual collections into one). The <code>Union</code> operators only applies to two collections. Any idea?</p>

Try
```
```
var it = GetTheNestedCase();
return it.SelectMany(x => x);
```
```
SelectMany is a LINQ transformation which essentially says "For Each Item in a collection return the elements of a collection".  It will turn one element into many (hence SelectMany).  It's great for breaking down collections of collections into a flat list.