Allow \u2e12 in variable names

[ChrisRackauckas]

This came up in https://github.com/JuliaDiffEq/ModelingToolkit.jl/issues/247 where u₁⸒₂ would be great name for a generated symbolic variable in a matrix, but it is not allowed with the current punctuation rules in the parser. I would advocate for allowing this inside of variable names since right now it doesn't have another use.

[JeffreySarnoff]

mᵢ⸒ⱼ⸒ₖ

[JeffBezanson]

Is there a latex sequence for this?

[JeffreySarnoff]

Not per se afaik. ($$m_{i,j,k}$$) ($_,$) though

[ChrisRackauckas]

Indeed the Latex we generate as output do m_{i,j,k}, but we can't have that as a variable name so we are trying to get as close as possible:

https://github.com/JuliaDiffEq/ModelingToolkit.jl/blob/v1.2.7/test/latexify.jl#L38-L51

Essentially what we're doing is taking in data and then spitting out the Latex for the physical laws that would generate the data, so we're trying to make our symbolic variables on the Julia side be symbols that are close to the Latex to make it easier to read and relate (since we're generating Julia code for the functions as well). What we're missing is a unicode subscript comma.

[StefanKarpinski]

It seems like it should be input as \_,<tab>.

[JeffreySarnoff]

There is also \u02cf 'ˏ' mᵢˏⱼˏₖ and \u201A '‚' mᵢ‚ⱼ‚ₖ (vs '⸒' mᵢ⸒ⱼ⸒ₖ using \u2e12) either is more comma-like and available in many more fonts (these lists do not include recently released fonts):
~10 font families supporting \u2e12 (mᵢ⸒ⱼ⸒ₖ)
~100 font families supporting \u201a (mᵢ‚ⱼ‚ₖ)
~40 font families supporting \u02cf (mᵢˏⱼˏₖ)

IBM Plex Mono has \u201a \u02cf, does not have \u2e12
FiraMono, Roboto Mono, Hasklig have \u201a, do not have \u02cf \u2e12

Given the lesser availability of \u2e12, one of the alternatives is preferable.

[knuesel]

u+201a actually looks too nice, i.e. it's too hard to distinguish from a regular comma: mᵢ‚ⱼ‚ₖ vs mᵢ,ⱼ,ₖ.

In a sense, that u+02cf looks a bit weird is an advantage, as it makes it clear that it's not a regular comma: mᵢˏⱼˏₖ vs mᵢ,ⱼ,ₖ (this is also true of u+2e12). This is what ModelingToolkit uses now.

There is also u+02cc "modifier letter low vertical line": mᵢˌⱼˌₖ. It is distinguishable from a regular comma, present in many fonts, and already allowed in variable names (this is all also true of u+02cf).