Implementing custom Options in Adobe AEM HTL (formerly known as Sightly)?

This question is about Options, a feature in Adobe's proprietary language HTL:

• https://helpx.adobe.com/experience-manager/htl/using/expression-language.html

• Expression Option Sightly

My simple question: Can I extend HTL by implementing my own custom Options? If so, how?

Yes, you can implement your own custom options for an HTL expression. You will need to implement a Filter and add it to the compiler.

But you shouldn't, as:

1. This is not a designed extension point. You will need to fork the implementation and modify it, thus becoming responsible for keeping it up-to-date further down the road.
2. You can most probably get the same results by using the public APIs (such as https://github.com/Adobe-Marketing-Cloud/htl-spec/blob/master/SPECIFICATION.md#221-use).
3. In case you think there's a very good reason for adding or modifying expression options, you can propose an HTL specification change (and also contribute the implementation in Sling).