Some day in my life i've created a tikz object using some tips from random parts of the internet and i've discovered the following:

`\def\foo{{a,b}}`

I don't have any example with it, but i'm preety shure that this is the way. It works like a vector, but i can't find any information of that structure. I've made a macro in my work using `xparse` package and i really wanted to use it inside those definitions, but some brutal erros get in the way.

What object is that? Just that i can search for it. And also, if someone could, there is a way to define something like the following?

```
\def\foo{{a,b}}
\usepackage{xparse}
\NewDocumentCommand{\mezzocommand}{m O{0}}{
    ${#1} + \foo[#2]$
}

```

So that `\mezzocommand{x}[1]` produce $x+b$? Sorry for the lackness of example, but i just need to know *how* to search for it.