Comment by lispm

> "Look at mathematical papers where people will introduce new brackets with new meanings"

Common Lisp has left brackets like {} and [] to the user (aka developer). It supports "reader macros", where the user can extend/supersede the syntax of s-expressions.

So, specialized tools/libraries/applications can introduce these brackets for their own use. Examples are embedded SQL expressions, notations for Frames (special objects, in kind of a mix of OOP and Logics), grammar terms, etc.

Thus it explicitly supports the idea of "people will introduce new brackets with new meanings".