[PD] curly braces in Pd

[Christof Ressi]

> the fact that curlies (that is: a pair of matching characters) have 
> been forbidden until now is a unique opportunity. 
Well, they have not been forbidden. You can not *type* them (yet), but 
they can appear in text files or in manually assembled symbols. In fact, 
they are properly displayed with [print] or list/symbol atoms, which 
suggests that they are already properly escaped when sent to the GUI. 
One could argue that someone who only uses libpd might not even be aware 
of this issue.

Curly braces have never been a part of special Pd symbols (semicolon, 
colon, dollar), so I'm sceptical that we can just give them some 
meaning. It would break lots of existing code that uses symbols with 
curly braces. For example, it would completely break the "purest_json" 
library.

However, we could use *escaped* curly braces. The escape sequences "\{" 
and "\}" are not defined, so we might use those for nested lists. I'm 
not saying it's pretty, though.

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Anyway, it's a bit silly that we still can't type curly braces, given 
how important they are for many use cases. I mean, I cannot even type a 
JSON string...

Christof