S-expression is a standard representation for syntax-free semantics. For example, PLT Redex [1] is a DSL for programming language semantics and built on top of Racket which uses S-expressions.

[1] https://redex.racket-lang.org/

Even Ruby `parser` gem uses S-expressions in their documentation: https://github.com/whitequark/parser/blob/master/doc/AST_FOR...