Documents can be represented as structures with
a hierarchical arrangement of text and non-text
nodes, where nodes are labelled by category names such as “paragraph” and “section”. Representing documents this way is a natural consequence of using the Standard Generalized Markup
Language (SGML) to encode the content and form
of documents [10, 11, 7]. SGML is widely used.
HTML, the encoding used for World Wide Web
documents, is an application of SGML ; although HTML is used to build hypertext networks
of documents rather than hierarchies, each document is itself a hierarchy with explicitly coded
links to build the network. The Text Encoding
Initiative uses SGML to encode complex texts [21,
4, 2]. Even documents that are not simple hierarchies can be represented using SGML .
Formally, documents represented in SGML or
HTML are trees with labelled nodes where the left
to right ordering of the offspring of a node is
significant. Any piece of text in the document can
be treated as a single labelled node, all leaf nodes
are text nodes (or empty) and any node with children is a structural or non-text node.
Families of structured documents:
There are many circumstances in which one structured document is a variant of another. A situation
in which a scholar deals with variants of a document can come about in two complementary ways.
In the first case, two or more existing documents
are known to be closely related. For example, there
may be several manuscript or printed versions of
an existing text; the details of the relationships
among the versions may be known or may have to
be inferred, but the variations among them are
small compared to the amount of text in common
among them. Or a text may exist in more than one
language with the original and the translations
being structurally identical. In this view of variant
texts, then, the documents are given, and the relationships among them are derived.
In the second case, a new document is derived
from an existing document by a known sequence
of transformations. If, for example, a text is being
created in a machine-readable form there may be
several versions produced for different audiences
(e.g., a text with minimal apparatus for students
and a more extensive apparatus for researchers),
or there may be interest in maintaining several
versions of a document that is being produced in
a cooperative manner by several authors . In
this setting, the changes made to the document can
be stored to produce a record of its evolution, and
to enable regeneration of any of its (intermediate)
In this second view of variant documents, where
the relationships (editing changes) are explicitly
given and the documents are derived, it is natural
to use hypertext structures [17, 23].
Software to handle variant texts in each of these
these two complementary views can be useful to
Editing or Co-authoring: When an author presents
a modified version of a document to an editor or
co-author, the two versions could be compared to
isolate only the changed components. A sophisticated display mechanism could highlight the differences. This would be a powerful editing tool in
either of the settings discussed above. If the documents are given, the differences to be displayed
must first be computed. If a sequence of transformations is given, in general some sequence of
changes must be applied to determine their net
Storing: Documents are often published in several
versions. A “document control system” might be
modelled on a source code control system to provide help in managing versions. One of the characteristics of such a system should be that it minimizes storage requirements by retaining only the
computed differences between versions of a document without having to store more than one complete document. This is a natural application of the
second view of document families.
Querying: If documents are stored in an archive
using a structured representation, a query against
the archive could also have a tree structure. The
document (or document fragments) that satisfy the
query would be those that most closely match the
query tree, given an appropriate metric for distan-
ce. This is a natural application of the first view of
We have studied the problem of managing document families using the first approach, where documents are given and relationships among them
are to be derived (or computed) .
The general approach to “edit distance” problems
(for strings or trees) in the literature has been to
define a sequence of primitive operations that can
be applied to one object to produce another, and
to define the distance between two objects as a
function computed on a sequence of such operations [19, 20, 22, 24, 25, 26]. In simple cases it can
be sufficient to determine the length of the sequence. More realistically and more generally, each
operation is assigned a “cost” that represents the
difficulty of making the indicated change to the
object. The cost could be thought of as the perceived unlikelihood of the change having arisen at
random in whatever process produced the changed
object. The problem is attacked by deriving an
algorithm to search for a sequence of operations
with minimal cost. (In effect, a linear programming algorithm is used to search for the desired
For string-to-string editing, the operations most
frequently considered are: insert a character, delete a character, and change one character into another. With trees, other operations are useful, including:
insertTree: Add an entire subtree (possibly
one leaf node) to the target tree.
deleteTree: Remove an entire subtree
(possibly one leaf node) from
the original tree.
insert: Add an internal node to the target
tree. delete: Remove an internal
node from the original tree.
change: Relabel a node in the original tree
with the label of a node in the
swap: Swap subtrees rooted at adjacent
swap with Swap subtrees rooted at
editing: adjacent siblings and edit the
subtrees in the original tree to the
corresponding subtrees in the
Determining the set of primitive operations that
can represent a change between versions is an
important aspect of this approach. In addition to
the kinds of changes just enumerated, machinereadable documents can have global changes applied (for example, change all occurrences of “colour” to “color”) in addition to those already described. Further work is required to incorporate
such operations if the set of transformations is
intended accurately to represent what might actually occur as documents evolve, rather than simply
to be a mathematical model of distance.
Algorithms for dealing with tree-to-tree correction
can be extended to structured documents. If the
documents are encoded in SGML, the tag name
and the attributes are taken together as the label on
In the complementary view, the computational
requirements appear more simple and straightforward: the changes encoded in the editing transactions must be applied to the original document to
form the derived document. Other more complex
computations are also useful, though, such as finding a minimal representation of a sequence of
changes. This requires the definition of a calculus
of basic editing operations, and an algorithm to
implement that calculus. For example, if as a document is being edited a word is inserted into a text
and later deleted, the minimal representation of the
changes should not include either the insertion or
A powerful electronic document processing system for research purposes should provide tools to
maintain document versions given an explicit set
of relationships among them, but should also include an algorithm for deriving a set of relationships from explicitly presented versions. For such
an algorithm to be useful, it must incorporate a
model of a rich set of document-editing primitives.
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