liability has been difficulty of understanding: users (and even researchers) steeped in the traditional paradigms sometimes struggle to break free of old assumptions. Yet if adopted on a wide scale, ZigZag’s so-called “ hyper-thogonal” structure offers the possibility of an ultra-flexible PKB, capable of adapting to all of a user’s needs.

 

the Role of transclusion

The term “transclusion,” first coined by Ted Nelson, 31 has been used in several senses. In general it means including an excerpt from one document into another, such that the including document maintains some kind of reference to the included document. The simplest form of transclusion would be a simply copy-and-paste operation wherein a link to the original source was maintained. A stronger form is when the transcluded content is not copied, but referenced. This can allow any updates to the referred-to document to be instantly seen by the referring one, or, in an even more sophisticated scheme, it allows the referring document to maintain access to the transcluded content as it originally appeared, and also any more recent versions of it. (The Xanadu project design was based on this latter formulation.)

In the context of PKBs, transclusion means the ability to view the same knowledge element (not a copy) in multiple contexts. It is so pivotal because it is central to how the human mind processes information. We think associatively, and with high fan-out. I may consider John, for instance, as a neighbor, a fellow sports enthusiast, a father of small children, a Democrat, an invitee to a party, and a hom-eowner who owns certain power tools, all at once. Each of these different contexts places him in relationship to a different set of elements in my mind. Without delving into psychological research to examine exactly how the mind encodes such associations, it seems clear that if we are to build a comprehensive personal knowledge base containing potentially everything a person knows, it must have the ability to transclude knowledge elements.

Bush’s original design of the me-
mex explicitly prescribed the transclu-
sion concept, for instance in his no-
tion of a “skip trail.” “The historian,”
he writes, “with a vast chronological
account of a people, parallels it with a
skip trail that stops only at the salient
items.” In this way, the full account
of the subject can be summarized in
a sort of digest that refers to select
items from the original, larger trail.
A modern example of transclusion is
Mediawiki,d the software used to host,
among other sites, Wikipedia. Its use
of template tags permits a source
page’s current contents to be dynami-
cally included and embedded within
another page.

Architecture

The idea of a PKB gives rise to some important architectural considerations. While not constraining the nature of what knowledge can be expressed, the architecture nevertheless affects matters such as availability and workflow.

File based. The vast majority of so-
lutions mentioned in this article use a
simple storage mechanism based on
flat files in a file system. This is true of
virtually all the mind-mapping tools,
concept-mapping tools, and note-
taking tools, and even a number of hy-
pertext tools (for example, NoteCards,
Tinderbox). Typically, the main “unit”
d http://www.mediawiki.org
of a user’s knowledge design—wheth-
er that be a mind map, a concept map,
an outline, or a “notebook”—is stored
in its own file somewhere in the file
system. The application can find and
load such files via the familiar “File |
Open…” paradigm, at which point it
typically maintains the entire knowl-
edge structure in memory.