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
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 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 DeRose (2004, Evaluation criteria) identifies several criteria for judging solutions to the overlap problem:
readability and maintainability, tool support and compatibility with XML, possible validation schemes, and ease of processing. Tag soup is, strictly speaking, not overlapping markup—it is malformed HTML, which is a non-overlapping language, and may be ill-defined. Some web browsers attempted to represent overlapping start and end tags with non-hierarchical Document Object Models (DOM), but this was not standardised across all browsers and was incompatible with the innately hierarchical nature of the DOM. HTML5 defines how processors should deal with such mis-nested markup in the HTML syntax and turn it into a single hierarchy. With XHTML and SGML-based HTML, however, mis-nested markup is a strict error and makes processing by standards-compliant systems impossible. The HTML standard defines a paragraph concept which can cause overlap with other elements and can be non-contiguous. SGML, which early versions of HTML were based on, has a feature called CONCUR that allows multiple independent hierarchies to co-exist without privileging any. DTD validation is only defined for each individual hierarchy with CONCUR. Validation across hierarchies is not defined by the standard. CONCUR cannot support self-overlap, and it interacts poorly with some of SGML's abbreviatory features. This feature has been poorly supported by tools and has seen very little actual use; using CONCUR to represent document overlap was not a recommended use case, according to a commentary by the standard's editor.
Within hierarchical languages There are several approaches to representing overlap in a non-overlapping language. The Text Encoding Initiative, as an XML-based markup scheme, cannot directly represent overlapping markup. All four of the below approaches are suggested. The Open Scripture Information Standard is another XML-based scheme, designed to mark up the Bible. It uses empty milestone elements to encode non-privileged components. To illustrate these approaches, marking up the sentences and lines of a fragment of Richard III by William Shakespeare will be used as a running example. Where there is a privileged hierarchy, the lines will be used.
Multiple documents Multiple documents can each provide different internally consistent hierarchies. The advantage of this approach is that each document is simple and can be processed with existing tools, but requires maintenance of redundant content and it can be difficult to cross-reference between different views. With multiple documents, the overlap can be analysed with data comparison and delta encoding techniques, and, in an XML context, specific XML tree differencing algorithms are available. Schmidt (2012, 3.5 Variation) recommends this approach for encoding multiple variants of a single text and to accept the duplication of the parts which do not vary, rather than attempting to create a structure that represents all of the variation present; further, he suggests that this alignment be performed automatically, and that misalignment is rare in practice. Example, with lines marked up:
With sentences marked up:
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