S-expressions. Lisp's s-expressions are a notation for hierarchical structuring of data. In Lisp, S-expressions can be code or data. At the heart of XML is also a notation for hierarchical structuring of data - tag pairs. As for programming languages and semantics - they come from applications built on top of the hierarchical notation. XSLT for example, is both XML and a programming language - a declarative programming language.
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