Document customizable binary operators

[JeffFessler]

I learned indirectly through comments on a PR that \otimes and \oplus and \odot can be extended as binary operators yet I cannot find this fact anywhere in the documentation. Search the docs for oplus yields nothing.
Somewhere there must be a list of which characters can be extended this way and it should be documented in the manual.
This PR is an attempt to get that ball rolling!

[mcabbott]

Sounds good to have. The complete list is here:
https://github.com/JuliaLang/julia/blob/master/src/julia-parser.scm#L29

(Note also that by postfix I think you mean prefix!)

I also can’t find where it is mentioned that things like *′(x,y) = x*y/2 also create infix operators. Is this in the manual?

[JeffFessler]

@mcabbott thanks for the link. I added it to the PR and also fixed the typo you mentioned.

I was unaware of the syntax you mentioned. Cautionary note to anyone who tries to reproduce it: the quote in *′(x,y) = x*y/2 is not an ordinary apostrophe! Seems like something that should be documented as well. Not sure where it would go.

[mcabbott]

It may be mentioned somewhere logical! But you are allowed any unicode decoration like ⊗ᵣ(x,y) = kron(y,x); /₂(x,y) = x/y/2.

The exact line I linked is the symbols parsed like * (in terms of precedence, and associativity), but there are more parsed like + or like -> etc.

[JeffFessler]

Thanks for the clarification. I've updated the PR to tell the story somewhat more generally.

[StefanKarpinski]

@JeffBezanson, can you give a read through and merge if you approve?

[JeffFessler]

@JeffBezanson, it's been a few weeks so I thought I'd make a gentle ping...