Corpus Encoding Standard - Document CES 1. Annex 10. Version 0.9.1 Last Modified 18 June 1996.


  Annex 10

  Overlapping hierarchies

Excerpt from "Background and context for the development of a Corpus Encoding Standard"

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The classical view of a document prepared for use in corpus-based research is one in which annotation is added incrementally to the original as it is generated. For example, a document containing type 1 markup might include the following:

Sentence boundary markup could be inserted directly into this document, resulting in the following:

However, this is not always possible. For example, if the document contains the following markup:

a likely segmentation into sentences would be

However, this is invalid SGML since the <s> and <q> tags are not properly nested.

This problem of overlapping hierarchies is a common one when applying SGML to certain complex descriptive situations, because the data model provided by SGML is that of an ordered labeled tree. When the phenomena to be recognized are independent of each other, they generally fail to nest regularly in a single hierarchy, requiring additional representations to be layered on top of SGML's basic structures. This occurs when there are multiple hierarchies, each to be applied to the same data, but where there are a well defined set of independent and hierarchical information types to be represented (as in the example above). Other common examples are the conflicts between typographic features (e.g., highlighting) and linguistic features such as sentence and word boundaries; and variant annotations (e;g. segmentations), which are generally non-hierarchical.

There are two basic approaches to this problem (with some variations):

To implement the first option for the example above, the markup would have to be

This encoding does not correspond, conceptually or typographically, with the content of the text, in which "better than ever. It is in fact in very good shape." is clearly regarded as a single quote. In general, it is often the case that overlapping hierarchies cannot be meaningfully broken. To preserve the intended relations among elements, additional markup in the form of cross-references would be needed to link the fragments of "split tags":

The drawback of this approach is that verification of added markup is very difficult for the secondary hierarchies, since any of the secondary elements can be interrupted at any time, and any sub-sequence of the possible contained sub-elements of an element is legal content for that element. Further, since the continued parts of elements are linked only by IDREFs or adjacency, standard SGML processing will not detect a wide variety of illegal secondary markup structures. This means that extra software tools will be required to verify that such data is correct as well as interpret the more complex markup in search and retrieval operations.

Typically, the second option, storing the hierarchies separately, is also implemented with a notion of primary and secondary markup. The primary markup is that recorded directly with the data in a single document, while secondary markup is associated with portions of the primary document by indirect reference. This method uses several SGML documents to represent a single logical document, but, because of that, can allow many parallel markup patterns for a base text, even ones added at a later time and not anticipated in the original tagging scheme. This method, in essence, treats the secondary markup schemes as annotations to the primary scheme.

For example, assuming the "base" document containing the quotation example above, the following markup for sentence boundaries would appear in a separate segmentation annotation document:

Base document:

Segmentation document:

(Note: In this example we use TEI notation involving ID references and character offsets to designate the target of the link.)

This is conceptually equivalent to inserting the markup for sentence boundaries as follows:

The separate markup strategy is in essence a finely linked hypertext format where the links signify a semantic role rather than navigational options. That is, the links signify the locations where markup contained in a given annotation document would appear in the document to which it is linked. As such the annotation information comprises remote markup which is virtually added to the document to which it is linked. In principle, the two documents could be merged to form a single document containing all the markup in each.


Another example where the SGML hierarchical view of documents is not convenient is for alignment of parallel documents, such as translations, transcription and recording of speech, etc. The alignment information is non-hierarchical; instead, it consists of a set of links between arbitrary regions of two or more documents. These links are the same as the kind of links used in hypertext systems to associate arbitrary pieces of documents.

For example, assume two parallel documents with regions delimited by <x> and <y> tags, respectively, which are to be aligned:

The regions are associated by means of a table indicating the correspondences (expressed here in TEI HyTime-based pointer notation):

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