@CarlLange It certainly is. See InputAliases

I even do with specialized boxes/formatting environments with InputAliases, largely through TemplateBox. All of this is possible to set at the stylesheet level

Hi folks, what is the difference between Wolfram Engine and Mathematica kernels? :-)

@KimJongUn Might be worth it to ask on the main site, maybe with an example of a similar thing. :)

@KimJongUn Wolfram Engine is kinda a kernel without a frontend and some other bits and bobs, and a different licensing structure.

@CarlLange Thank you. Understood.

@b3m2a1 Thanks, that's really helpful. I've almost never looked at the stylesheet stuff because it seemed very 2004 and I didn't want to give myself a headache. Are you aware of a handy QA somewhere to explain how to do this for all notebooks?

@kirkus That's probably a good idea, but I work on linux and I don't know if there's a nice cohesive option there. I actually use a custom keyboard for which I can write the firmware (using QMK qmk.fm) so it's really my own fault that | is hard to reach

Suppose I could actually make some keys that just input <| and |> now that I think about it...

Well, I guess that only took 2 hours of my life away... but now I can write turbofishes easily! <||->|>

And to my question earlier: you can set it for all notebooks in the options inspector.

I am not good at pattern matching.

@KimJongUn I don't have Mathematica available right now but perhaps something like Join @@@ f[...] or Flatten /@ f[...]. If you need to do it with pattern matching, perhaps f[...] ./ f[x_] :> f[Flatten[x]]

@C.E. Thank you. The last one is promising. I am trying it now.

It works. thanks!

Why using [] is prohibited?

does not work

mathematica is symbolic but does not allow []

@KimJongUn I don't understand what you are trying to say with that code. All Mathematica code is a single expression of the form head[subexpr1, subexpr2, ...]. Lots of books on Mathematica begin with a sentence to this very effect. Have a look at FullForm to see that any Mathematica code can be written in this way. Now, [x__] is supposed to be a subexpressions of Rule (->) but it clearly is not a an expression, because it does not have the format head[subexpr1, subexpr2,...]

@C.E. OK. Make sense. Thank you very much!