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The term hypertext was coined by Ted Nelson in the early 1960s. In his understanding hypertext stands for non-sequential writing. To the reader hypertext offers several different branches to assemble the meaning behind the written text. It is not possible to articulate orally two ideas at the same time. They have to be put in sequence to be properly told to the recipient of the message. The linear structure of printed books is just deduced from the linear structure of speech. The way such coherent texts are created by humans is far from linear. It is associative, as also Vannevar Bush has said twenty years before. Computers should be used to support the deep structure of thinking. Hypertext should free the author from the need of linearizing her text.

Xanadu (cf. 2.1.2) is Ted Nelson’s own attempt for a hypertext system. It is more of a framework than a working program, although several aspects of Xanadu have exceeded the level of working prototypes. The incontestable merits of Xanadu is the influence it took on nearly all hypertext systems to come.

2.1.2 Xanadu

in Vision and Reality of Hypertext and Graphical User Interfaces

Ted Nelson strives for just a «decent writing system», as he says in his book Computer Lib / Dream Machines [Nelson 74, p. DM 59]. His central idea is a docuverse, a universe of documents, where a new form of literature can proliferate without the limitations of a linear medium like the printed book. A condensed version of his vision can be found in the preface of the 1993 edition of Literary Machines [Nelson 93, p. 10],

At your screen of tomorrow you will have access to all the world’s published work: All the books, all the magazines, all the photographs, the recordings, the movies. (And to new kinds of publication, created especially for the interactive screen.)

You will be able to bring any published work to your screen, or any part of a published work.

You will be able to make links – comments, personal notes, or other connections – between places in documents, and leave them there for others (as well as yourself) to follow later. You may even publish these links. […]

Any document may quote another, because the quoted part is brought – and bought – from the original at the instance of request, with automatic royalty and credit to the originator.

Xanadu is an continuously ongoing project since 1960. The system serves for Nelson as a framework to implement the concepts that constitute hypertext in the direction depicted above. Parts of this long term research project finally became public under the Open Source model in 1999. Other aspects have been demonstrated as mockups and working prototypes.

Fig. 2.4 A mockup showing Parallel Textface™ for the Xanadu System, 1972

The fundamental problem of hypertext is, according to Ted Nelson in Parallel Visualization: Transpointing Windows [Nelson 98a], the ability to see connections side by side. The links themselves should be visualized on screen. For that reason early versions of Xanadu are designed around the concept of Parallel Textface™. The mockup shown in Fig. 2.4 uses a cardboard and a celluloid picture to simulate a computer screen. It illustrates how connections between two columns of text should appear. The lines ought to be moving as either of the columns is scrolled by the user.

The correspondence between Xanadu’s parallel text columns and Memex’ two adjacent screens (as shown in Fig. 2.3) is obvious.

With the advent of windows of the graphical user interface the concept of Parallel Textface™ evolves to Transpointing Windows. Therefore the lines need to cross the window borders and bridge to the next window.* *The Web editor Adobe GoLive implements such a behavior for the creation of html hyperlinks, Adobe GoLive’s Point & Shoot: an interface technique for creating hyperlinks [Müller-Prove 99].

Ted Nelson has a much deeper understanding of hypertext systems than just the user interface issues. He requires four fundamental qualities: Consistent hyperlinks between content, robust re-use of content, support of versioning and support for parallel documents.
The first two principles touch on connecting any two pieces of content, either by hyperlinking or by transclusion respectively. Transclusion means a sort of quotation without copying the content but borrowing it from the original source. None of these connections should ever break. This calls for a persistent identification mechanism for content, and the content itself should never be deleted from a xanalogical hypertext system. But of course changes are permitted. Hence the system has to cope with several revisions of the same content – versioning is the third principle. And finally, what makes up a parallel document? A flock of versions and variations of a single theme or idea is called a parallel document.

One of Ted Nelson’s Examples of Parallel Documents [Nelson 98b] takes Hamlet. Hamlet, he says, is not just one fixed piece of text. Shakespeare himself has created several versions. The play has been translated to other languages and to other media, consider all the Hamlet movies acting Lawrence Olivier, Richard Burton, Mel Gibson and Kenneth Branagh. Hamlet has also been studied and therefore been annotated by English language and literature scientists. If one refers to “Hamlet” one means the wholeness of documents that have been created under the same idea.

Another example that might sound less offside is about email. It is inspired by Alan Cooper’s book The Inmates Are Running the Asylum [Cooper 99, p. 61]. A conversation via email produces a bunch of single mails. It is desirable that the system can treat all related mails as part of a sequence – as one parallel document.

The four xanalogical principles above are discussed in Xanalogical Structure, Needed Now More than Ever: Parallel Documents, Deep Links to Content, Deep Versioning and Deep Re-Use [Nelson 99a]. Many, if not all existing hypertext systems, fall short if they get evaluated against Nelson’s criteria.

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