Book-like indices are a notable example. A book index is a list of topics with references to pages. A topic may very easily overlap with another topic and all i

Book-like indices are a notable example. A book index is a list of topics with references to pages. A topic may very easily overlap with another topic and all indexing tools imply that. For example, XSL-FO is an XML-based notation, so in general it is strictly hierarchical: a tag encloses other tags. For index entries, however, it uses disjoint tags like `<index-range-begin>` and `<index-range-end>` linked via the `id/ref-id` attributes.