FoLiA: Format for Linguistic Annotation - FoLiA is a rich XML-based annotation format for the representation of language resources (including corpora) with linguistic annotations. A wide variety of linguistic annotations are supported, making FoLiA a useful format for NLP tasks and data interchange. Note that the actual Python library for processing FoLiA is implemented as part of PyNLPl, this contains higher-level tools that use the library as well as the full documentation, validation schemas, and set definitions
This proves to be a bit harder than expected: http://stackoverflow.com/questions/39624122/how-to-require-mixed-element-to-be-non-empty-in-relaxng