My preceding post promised to compare experiences implementing the Canonical Text Services protocol with three equivalent data structures for text: trees (formatted in XML), tables, and graphs (expressed in RDF). Before turning to the first of these data structures, however, I should expand briefly on the comment in that post that, in developing the CTS protocol, "we relied heavily on the OHCO2 model." More precisely, I mean that we developed CTS so that it fully expresses the semantics of OHCO2: hence the title of the present post.
The CTS protocol uses CTS URNs to cite passages of texts. The semantics of CTS URNs by themselves give us two of the four OHCO2 properties, since a CTS URN specifies where in a citation hierarchy a passage of text is situated, and where in a hierarchy of versions a particular version is situated. A URN like urn:cts:greekLit:tlg0012.tlg001.msA:9.119 for example, refers to a passage set in a version of the Iliad (the work tlg0012.tlg001) identified as msA (i.e., the Venetus A manuscript), and refers to a citable line (119) contained within a citable book (9).
The remaining two OHCO2 properties are provided by a pair of CTS requests. The GetPrevNext request places a passage within an ordered sequence; the GetPassage request returning the contents of the passage supports a mixed content model.
After some initial experience developing applications built on CTS, Chris Blackwell suggested that it would be convenient for developers to have both GetPrevNext and GetPassage information available via a single request. We introduced the CTS GetPassagePlus request for just this purpose. His intuition is now gratifyingly justified by the observation that the GetPassagePlus request tells us everything about a cited passage of text that the OHCO2 model guarantees.