LMNLAn acronym for Layered Markup and Annotation Language. It allows overlapping ranges, has structured annotations (attributes), namespaces, and layers. (Layers can be used to provide different views of the same document and add to other layers.) It is similar to Ted Nelson's parallel markup (which was to be used in Xanadu).
Actually, LMNL demonstrates a problem with parallel markup: nesting tag-pairs does not have to be an inclusion issue. In some cases, two pairs of tags over the same region may be "concurrent" instead of one inclusive of the other. Furthermore, the ordering of their concurrency may have meaning as well.