Add narrow emptyset symbol

[mkorje]

I've gone with the modifier .alt for now, matching things like pi.alt etc. Please share any other ideas you might have

[Enivex]

The description in https://www.unicode.org/Public/17.0.0/ucd/StandardizedVariants.txt is very explicit

2205 FE00; zero with long diagonal stroke overlay form; # EMPTY SET

I think the name should reflect this. Such as emptyset.zero.

[MDLC01]

I personally prefer .alt over .zero. .zero seems needlessly specific to me: I don't think of the "zero" variant as an actual zero, and .alt is already an established modifier for alternate versions of symbols. For example, phi.alt could be more specifically phi.circle because it contains a circle, but that would also be over-specific.

What I'm wondering is whether the narrow variant should be more easily accessible. I remember someone suggesting many months (years?) ago that one of emptyset or nothing could be used to access the variant. Or maybe we could add a separate name for it, such as null or void?

[Enivex]

I personally prefer .alt over .zero. .zero seems needlessly specific to me: I don't think of the "zero" variant as an actual zero, and .alt is already an established modifier for alternate versions of symbols. For example, phi.alt could be more specifically phi.circle because it contains a circle, but that would also be over-specific.

That's not really a like for like comparison. Unicode is not particularly prescriptive about what the two phi symbols should look like (though most fonts align along general lines).

The standardized variants, however, are explicitly intended to codify particular versions of the symbol. It's not just an alternate version, but specifically the version of the empty set that looks like the number zero with a slash through it.

It's not like #127 makes the symbols inter.alt and union.alt.

What I'm wondering is whether the narrow variant should be more easily accessible. I remember someone suggesting many months (years?) ago that one of emptyset or nothing could be used to access the variant. Or maybe we could add a separate name for it, such as null or void?

In my experience the narrower empty set is relatively uncommon in modern literature. It's widely regarded as a mistake that it was made the default one in TeX. None of the various modified empty sets have the narrower variant either. For this reason I don't see a hurry to make it more easily accessible.

[MDLC01]

It's widely regarded as a mistake

Do you have sources about that? If this is true and the narrow variant originates from a mistake and isn't used much, then I would agree with not making it more accessible.

It's not like #127 makes the symbols inter.alt and union.alt.

My feeling is that the narrow emptyset variant is really the alternative form of the emptyset variant, while the serif variants of union and intersection symbols are somewhat more niche and specific. But I don't have a strong opinion and I'm fine with .zero if other people agree that this is better than .alt.

[knuesel]

I also have a preference for .zero as it is more explicit, but more importantly because there's a third variant in use: the slashed O letter (which is the symbol I get when browsing https://mathworld.wolfram.com/EmptySet.html for example). It's not as common as the other two, but still, it means there's more than one alternative.

There's a nice overview of the subject in the Unicode proposal for the slashed zero variant: https://www.unicode.org/L2/L2015/15268-slashed-zero.pdf