Non-hierarchical interaction of overlapping document markup entities
In markup languages and the digital humanities, overlap occurs when a document has two or more structures that interact in a non-hierarchical manner. A document with overlapping markup cannot be represented as a tree. This is also known as concurrent markup. Overlap happens, for instance, in poetry, where there may be a metrical structure of feet and lines; a linguistic structure of sentences and quotations; and a physical structure of volumes and pages and editorial annotations.
History [ edit ]
The structural differences between multiple editions of Frankenstein have been analysed with overlapping techniques.
The problem of non-hierarchical structures in documents has been recognised since 1988; resolving it against the dominant paradigm of text as a single hierarchy (an ordered hierarchy of content objects or OHCO) was initially thought to be merely a technical issue, but has, in fact, proven much more difficult. In 2008, Jeni Tennison identified markup overlap as "the main remaining problem area for markup technologists". Markup overlap continues to be a primary issue in the digital study of theological texts in 2019, and is a major reason for the field retaining specialised markup formats—the Open Scripture Information Standard and the Theological Markup Language—rather than the inter-operable Text Encoding Initiative-based formats common to the rest of the digital humanities.
Properties and types [ edit ]
A distinction exists between schemes that allow non-contiguous overlap, and those that allow only contiguous overlap. Often, 'markup overlap' strictly means the latter. Contiguous overlap can always be represented as a linear document with milestones (typically co-indexed start- and end-markers), without the need for fragmenting a (logical) component into multiple physical ones. Non-contiguous overlap may require document fragmentation. Another distinction in overlapping markup schemes is whether elements can overlap with other elements of the same kind (self-overlap).
A scheme may have a privileged hierarchy. Some XML-based schemes, for example, represent one hierarchy directly in the XML document tree, and represent other, overlapping, structures by another means; these are said to be non-privileged.
Schmidt (2012) identifies a tripartite classification of instances of overlap: 1. "Variation of content and structure", 2. "Overlay of multiple perspectives or markup sets", and 3. "Overlap of individual start and end tags within a single markup perspective"; additionally, some apparent instances of overlap are in fact schema definition problems, which can be resolved hierarchically. He contends that type 1 is best resolved by a system of multiple documents external to the markup, but types 2 and 3 require dealing with internally.
Approaches and implementations [ edit ]
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