Multiple pipelines that merge within a sklearn Pipeline? : <p>Sometimes I design machine learning pipelines that look something like this:</p>

<p><a href="https://i.stack.imgur.com/2mw5F.png" rel="noreferrer"><img src="https://i.stack.imgur.com/2mw5F.png" alt="Example pipeline"></a></p>

<p>Normally I have to hack these "split" pipelines together using my own "Combine Features" function. However, it'd be great if I could fit this into a sklearn Pipeline object. How would I go about doing that? (Pseudo-code is fine.)</p>
