Add \bbsemi as a binary operator

[tkf]

; is used as the composition operator in "diagrammatic order" (Category (mathematics) - Wikipedia). Although we can't use the normal semicolon as a binary operator in Julia, we can include ⨟ (\fatsemi) as a binary operator.

A big thanks to the post by @schlichtanders in Discourse: https://discourse.julialang.org/t/add-composition-operator-unicode-2a1f-for-flipped/34436

[stevengj]

What’s the difference between U+2a1f and U+2a3e?

[tkf]

Formal Specification— Z Notation— Syntax, Type and Semantics (http://www.open-std.org/jtc1/sc22/open/n3187.pdf) says

schema composition ‘o9’ and the mathematical toolkit character relational composition ‘o9’ are different Z characters;

(o9 is rendered as o on top of 9 in the PDF)

The motivation discussed in the aforementioned discourse thread was to define f ⨟ g = g ∘ f that is presumably primarily used for functions. So, IIUC, I think relational composition (2A3E) makes more sense (see Table 14). @schlichtanders @jw3126 What do you think?

As two characters are pretty much identical, I think omitting schema composition (2A1F) makes sense. This is compatible with that we don't have, e.g., \setminus (∖).

Another consideration may be to see if there are fonts that have only one of the characters. But so far I find that it's either both of them are rendered or none of them are rendered (though maybe it's just my editor and terminal are finding fallbacks).

[schlichtanders]

What’s the difference between U+2a1f and U+2a3e?

From simple inspection of the unicode alternatives at this page https://unicode.org/charts/PDF/U2A00.pdf

I find 2A1F much more appealing visually (compared to its counterpart 2A3E). In addition, also its latex name is lovely self-descriptive \fatsemi

[tkf]

It seems there are multiple ways to enter certain symbols. So I hope it's OK to add both \fatsemi and \bb; (as how it's implemented in the current PR).

julia> length(unique(keys(REPL.REPLCompletions.latex_symbols)))
2495

julia> length(unique(values(REPL.REPLCompletions.latex_symbols)))
2475

Re: 2A3E vs 2A1F, I don't really mind which one is chosen. It looks like 2A1F is rendered as a bigger symbol so maybe it's better for readability.

[cormullion]

There's a bit of variety in the commonly used fonts:

Z NOTATION SCHEMA COMPOSITION 2A1F
Z NOTATION RELATIONAL COMPOSITION 2A3E

(But a mild vote against "fatsemi" which is far from a "lovely" name... :))

[JeffBezanson]

I think \bbsemi would be more consistent with other latex names than \bb;.

[tkf]

I replaced \bb; with \bbsemi. Since no one seems to be opposing U+2a1f, I also replaced u+2a3e with u+2a1f.

Can this be merged? Is there anything else to be addressed?

[tkf]

I removed \fatsemi.

Note that people want to use this alias can simply do:

julia> using REPL

julia> REPL.REPLCompletions.latex_symbols["\\fatsemi"] = "⨟"