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I agree with you, I perceive it to be really difficult to change existing code to fit new things in. There's all interactions between things.

The source code of the widgets we use everyday - such as text editing or graphical editors are or your web browser's Javascript engine, are enormous.

'Just do X, too' is really difficult with what is already there.

Here you go (not my work, but it's interesting): https://github.com/yzyzsun/CP-next

Do you have an example of the expression problem that can't (yet) be solved in today's programming languages, but could theoretically be solved?

I solved it in mine. I should write a blog post.