Add operator @ for matrix multiplication

[DanielFath]

@lordan

I merely wanted to highlight that there are options other than cryptic operator symbols. (another example: this classic paper)

That paper is an April fools.

[felix.s]

@toc

For typing symbols directly, compose keys are an excellent solution. I don't have € on my keyboard but typing <compose> C = does the job. This can get quirky quickly.

AltGr+5 works for me too. It’s even appropriately marked on some keyboards.

@toc

So a better solution at the language level is having common escape sequences, translated by rustfmt/rustc/IDE. I don't have ⊂ on my keyboard but (in lean) \subset gets translated just fine.

No need to build it into the language.

What's the best way to actually "type" special UTF-8 chars?