> But if I want something that is able to express data structures customized by myself, usually with hierarchical data that can be verified for validity and syn

> But if I want something that is able to express data structures customized by myself, usually with hierarchical data that can be verified for validity and syntax (XML Schemas or old-school DTD), what other options are there? S-expressions work great. Syntax checking is far simpler, and validity checking is hence something you can roll yourself (and writing an S-expression schema checker ain't tough).