Lloyd Rutledge, Lynda Hardman, Jacco van Ossenbruggen and Dick C. A. Bulterman “ Implementing Adaptability in the Standard Reference Model for Intelligent Multimedia Presentation Systems” , Proceedings of Multimedia Modeling 98, October, 1998.
Implementing Adaptability in the Standard Reference Model for Intelligent
Multimedia Presentation Systems
Lloyd Rutledge, Lynda Hardman, Jacco van Ossenbruggen* and Dick C.A. Bulterman
CWI (Centrum voor Wiskunde en Informatica), Amsterdam, The Netherlands
*Vrije Universiteit, Amsterdam, The Netherlands
Abstract
This paper discusses the implementation of adaptability in
environments that are based on the Standard Reference
Model for Intelligent Multimedia Presentation Systems.
This adaptability is explored in the context of style sheets,
which are represented in such formats as DSSSL. The use
of existing public standards and tools for this
implementation of style sheet-based adaptability is
described. The Berlage environment is presented, which
integrates these standards and tools into a complete
storage-to-presentation hypermedia environment. The
integration of the SRM into the Berlage environment is
introduced in this work. This integration illustrates the
issues involved in implementing adaptability in the model.
1 Introduction
Separation of structural and style information has
long been commonplace for text, and can also be found in
many hypertext models. The Dexter hypertext reference
model separates the structural information of the storage
layer from the style and layout information encoded in the
presentation specifications [14]. The AHM (Amsterdam
Hypermedia Model) extends Dexter into hypermedia by
accounting for timing, more complex aspects of user
interaction and how these should be conveyed to the user
[12]. In most hypermedia design models, including RMM
[19] and HDM [8], the two types of information are
designed during different phases of the design process.
Intelligent Multimedia Presentation Systems (IMPS)
deal with the dynamic creation of multimedia presentations
optimally geared towards the needs of a user. In broad
terms, the created presentation should define what is
presented to the user (the content), where it is presented
(the spatial layout) and when it is presented (temporal
layout). Given a collection of user goals and availability of
resources these three aspects leave open an immense
number of possible presentations. An IMPS is a reasoning
system aimed at selecting the optimal one.
The Standard Reference Model for Intelligent
Multimedia Presentation Systems (SRM) [1] specifies the
decomposition of this process into well defined layers. It
can be used as a basis for discussing and comparing
different systems that adapt to the user and his or her
presentation environment. The SRM can also be used in
guiding the development of such systems. The Berlage
environment was developed as prototype for exercising
issues of adaptability in hypermedia [22]. For this paper
the Berlage environment was modified and extended to
follow the design of the SRM and to incorporate some of
its functions. The Berlage environment is built of a
collection of related public formats and tools for
hypermedia and document processing.
Because of the diversity of multimedia user
communities and of the functions and processing
approaches involved, no one format has been found that
applies to all multimedia environments. As a result, many
different formats for hypermedia exist, each with a
different scope. Some focus on particular document sets or
user communities and thus are quite detailed and not
widely applicable. Others focus on different sets of shared
functions between communities of users, thus providing
common frameworks but not complete environments.
Some focus on details of the final hypermedia presentation,
while others define more general structure of documents
that may be presented in a variety of different manners.
The ISO standard HyTime (Hypermedia/Time-based
Structuring Language) specifies the representation of
hypermedia documents in a presentation-independent
format [15][7]. HyTime is defined as a subset of SGML
(Standard Generalized Markup Language) [18][9], which
defines the structure of electronic documents in general. A
related language that is also defined as an SGML subset is
XML (Extensible Markup Language), which provides a
common framework for documents from different
applications on the Web [2].
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The ISO standard DSSSL (Document Style
Semantics and Specification Language) encodes the
transformation from SGML documents to formats that
present them [17]. Thus, DSSSL systems can accept XML
and HyTime as input. The use of DSSSL with HyTime was
recently made easier with the release of the second edition
of HyTime, which contains new facilities for use with
DSSSL.
SMIL (Synchronized Multimedia Markup Language,
pronounced “smile”) is a new W3C proposed
recommendation for final-form hypermedia presentations
distributed on the Web [14]. With W3C’s promotion and at
least two publicly available SMIL players, it is expected
that SMIL will be a widely-used means of distributing
hypermedia documents. SMIL is defined as a subset of
XML. Thus, DSSSL can encode transformations that
output SMIL.
The issues discussed in this paper are illustrated with
the Berlage hypermedia authoring and browsing
environment [22]. This environment incorporates the use
of HyTime to represent hypermedia documents in a format
independent of their presentation. It also incorporates the
use of DSSSL to specify the different mappings of these
documents to their final presentations. Finally, Berlage
generates and displays presentations encoded in SMIL.
The Berlage environment consists of publicly available
tools to demonstrate how such environments can be readily
implemented on a wide scale.
These issues are also illustrated with an example
application of the Berlage (named after H.P. Berlage, the
architect of several buildings in Amsterdam) environment.
This application is about the city of Amsterdam, The
Netherlands, and is called Fiets (Foundation for Interactive
Electronic Touring Systems, or fiets {pronounced “feets”},
the Dutch word for “bicycle” and generally the preferred
means of personal transportation in Amsterdam) [22]. Fiets
provides geographic and historic information about
Amsterdam.
This paper first describes adaptability as
implemented by style sheets in existing formats, tools and
environments. Then the Berlage environment and its
means of processing style sheets using these formats and
tools is discussed. An overview of the SRM is provided.
Finally, the use of the SRM in the architecture of the
Berlage environment is presented. This use of the SRM
illustrates the issues of adaptability that arise from its
implementation.
2 Use of style sheets for adaptability in
hypermedia
In this paper adaptable hypermedia is hypermedia
that uses external intervention to be adapted for
presentation. This contrasts with adaptive hypermedia,
which adapts autonomously for presentation. Often
adaptive hypermedia involves the author establishing what
the varying circumstances of presentation are when the
document is written and accounting for these possibilities
at that time [3]. Adaptable hypermedia typically requires
more processing than adaptive hypermedia, especially at
presentation time. Adaptable hypermedia also requires
maintaining a division between the document itself and
how it is presented. However, adaptable hypermedia is
usually more versatile than adaptive hypermedia, being
able to account for presentation circumstances the author
may not be able to predict. This paper concerns itself
primarily with adaptable hypermedia. The distinction
between an adaptable hypermedia document and its
presentations is maintained with style sheets.
While the SRM defines a general processing model
for dynamic presentation generation and adaptability,
existing formats and tools based on style sheets currently
provide an infrastructure for performing similar functions.
The difference is that style sheet-based environments often
do not have dynamic adaptation. Instead, static
presentations are often generated that are tailored for
circumstances that are known at generation time. This
section discusses the DSSSL style sheet format and how it
handles presentation generation and adaptability. Later this
paper applies this discussion to implementing the SRM.
DSSSL is a Scheme-like language that describes how
an SGML document is transformed into another SGML
document or into a non-SGML format. Because HyTime
documents are SGML documents, any HyTime document
can be transformed by DSSSL. A DSSSL program is
typically called a style sheet. The separation of style from
structure and content enforced with the distinction between
DSSSL and SGML/HyTime facilitates the creation of
particular styles by the author that can be applied to
documents of the same document set. Note that although
the term style sheet is used, DSSSL can be
used for more general, non-style transformations.
The design of typical DSSSL usage is shown in
Figure 1. This diagram shows how an SGML document is
processed with an accompanying style sheet by a DSSSL
engine. The DSSSL engine processes the style sheet to
generate the desired transformation of the source document
into the presentation format. How this style sheet-based
architecture was implemented in the Berlage environment
is described later in this paper. Also described later in this
Implementing Adaptability in the Standard Reference Model ... Multimedia Modeling 98
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paper are the extensions to this architecture, and their
implementation in the Berlage environment, that are
required to implement dynamic generation of
presentations.
A hypermedia style sheet language needs to be able
to define a mapping from the source hypermedia document
model to the model of the target format. This can be the
final-form model of the presentation (i.e. the model of the
play-out environment) or another intermediate document
model. In adaptive environments, style sheet conversion
might also adapt to user characteristics (e.g. level of
expertise) or to changing system resources (e.g. network
bandwidth). A hypermedia style sheet could be used to
indicate how to deal with limited resources (e.g. by
specifying alternatives or quality of service negotiation
protocols) on different platforms, thus making the
document source independent of platform-specific details.
The DSSSL formatting process, for which publicly
available tools exist [6], does such structural
transformations. For documents with purely text content,
DSSSL users can transform one SGML document type into
another. The same process can also be applied to
hypermedia document formats based on SGML. For
instance, since SMIL documents are SGML-conforming
documents, this extension can be used to generate SMIL
documents from any other SGML-based hypermedia
document format. One can use this to generate SMIL
presentations from more abstract HyTime-encoded
hypermedia documents. The Berlage environment uses this
approach to translate SMIL to MHEG [24]. By employing
reusable DSSSL libraries [16], the inner details of
languages such as SMIL or MHEG can be hidden from
individual style sheets. Such libraries would hide the inner
details of the target format from the style sheet.
Input
SGML
Document
and DTD
Document
in
Present’n
Format
Document
Player
DSSSL
Engine
DSSSL
Style
Sheet
Present’n
Status
Introduction
of Dynamics
Figure 1: Typical Usage of DSSSL and the
Introduction of Dynamics
3 The Berlage environment
This section describes how existing publicly
available standards and tools are used in building the
Berlage environment. First, the standards HyTime and
SMIL are described, as is how they apply, respectively, to
the storage and presentation of hypermedia. This is
followed by a description of the Berlage environment and
how it uses these standards and publicly available tools that
process them to make a complete hypermedia authoring
and presentation system. How the Berlage environment
was extended to implement the SRM is described in the
next section.
3.1 HyTime
HyTime is an ISO standard for representing
presentation-independent hypermedia data. It is built upon
SGML, which provides the basic structuring information
that applies to document data in general. HyTime adds
more complex structuring constructs and attaches
hypermedia semantics to certain patterns of composites of
this structure. The basic hypermedia semantics that
HyTime represents include hyperlinking, which establishes
descriptive relationships between document objects, and
scheduling, which puts document objects in coordinate
systems that can represent spatial and temporal structure.
HyTime and SGML are generally intended for
encoding documents that are presentation-independent.
They can apply to a wide variety of presentation situations
but do not themselves represent particular presentations.
HyTime and SGML documents typically must be
processed into a different format appropriate for final
presentation.
HyTime and SGML are meta-languages. They
encode not only individual documents but also the
document sets to which they belong. A document set is
defined by an SGML DTD (document type definition). An
individual document conforms to a particular DTD. A
DTD defines a specific syntax, in terms of SGML
constructs, that its documents must follow. HyTime
inherits from SGML the use of DTDs to define individual
document sets.
With the document content and general structure
being established by HyTime, the author of DSSSL style
sheets can focus on the desired style of presentation, the
mapping of the navigational interface, and the adapting of
the presentation to particular environments and individual
users.
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3.2 SMIL
Standard text editors are used to create Berlage
documents, DTDs and style sheets. A Berlage document
and corresponding style sheet are input to a DSSSL engine
and a SMIL document is generated. The DSSSL engine
used in the Berlage environment is Jade, which is publicly
available [6]. This SMIL document is then presentable
across the Web with GRiNS or other SMIL players.
SMIL is a format representing hypermedia
presentations on the Web. It incorporates basic hypermedia
principles such as spatial layout, temporal composition,
synchronization and navigational hyperlinking. It also has
constructs that adapt presentations to the characteristics of
individual environments and users. SMIL specifies the
display of multiple media items in a coordinated fashion:
displaying visual items and related locations on the screen,
and synchronizing the timing of the presentation of these
media items. SMIL specifies the display of a navigational
interface that allows the user to select what portions of the
presentation to currently display.
SMIL has an easy-to-author format whose syntax
resembles HTML. Since SMIL documents are XML
documents, they are SGML documents, and thus are
readily processed as output, and as input, of DSSSL
transformations.
4 Implementing the SRM in the Berlage
environment
The SRM components are illustrated in Figure 3. The
SRM divides dynamic presentation generation into two
areas: generation process and knowledge server. The
generation process performs the run-time generation of the
presentation based on the user’s interaction, the history of
the presentation and information provided by the
knowledge server. The knowledge server stores and
provides long-term instructions and information that apply
to multiple presentations at any point in their run. As such,
the generation process is the active component of the SRM,
and the knowledge server provides the data that gets
processed.
Before the incorporation of the SRM, the Berlage
environment created only static presentations. With the
SRM incorporation performed for this paper, the Berlage
environment can adapt the presentation during the
presentation itself. The dynamic generation of DSSSL
code required by the SRM implementation necessitated
additions and modifications to the Berlage environment
architecture. These additions are shown in Figure 2. The
primary new component added is the http server. The
server enables the Berlage environment to keep track of the
3.3 The Berlage environment
Berlage is a hypermedia environment that uses
publicly available standards and tools. The Berlage
environment was introduced in previous work [22]. A
diagram of the environment design is shown in Figure 2.
HyTime is used by Berlage to represent stored
hypermedia data. SMIL is used to encode the hypermedia
presentation of Berlage documents. DSSSL is used in the
Berlage environment by authors to encode presentations of
storage documents. A large document can be written once
for long-term storage, and it can be presented in many
different ways without re-editing the original document.
(Fiets)
Document
and DTD
Text
Editor
SMIL
Document
Jade
GRiNS
Player
DSSSL
Style
Sheet
http
Server
Present’n
Status
SRM Implementation
Figure 2: Berlage Environment Design with SRM Implementation
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user’s interaction with the presentation and with each
interaction generate a new presentation state in the form of
DSSSL code which can be processed with the style sheet.
The environment then calls the Jade DSSSL engine to
again process the document with the DSSSL style sheet,
which includes by reference the newly generated
presentation state code. This results in the creation of new
SMIL code, which is then passed back to the SMIL player.
To enable Berlage to keep track of the user
interaction, the destination of all navigational hyperlinks in
the SMIL code is specified with URL code that the http
server recognizes. These URL addresses are in the format
used by HTML forms. Fields and values for those fields
are set in the URL to convey to the server what the status
of the presentation was and what choice the user selected
from among those that were available. The server can then
update its internal data store to properly represent the
presentation’s state. Next it generates the DSSSL
presentation state code for inclusion in the next Jade
processing of the document and its presentation. This
generated DSSSL code includes data that enables the style
sheet to define SMIL hyperlink destination URLs so that
the server will recognize them when they are activated and
respond appropriately.
Goal
Formulation
Application
Control
Layer
Application
Expert
Generation Process
Context
Expert
Design
Layer
User
Expert
Realization
Layer
Design
Expert
SMIL
GRiNS
Knowledge Server
Content
Layer
Presentation
Display Layer
User
Berlage Equivalents
in Italics
Figure 3: The SRM and its Components
The SRM generation process is divided up into
layers. The control layer determines how the goals of the
presentation should be met and makes sure that they are. In
the content layer, a rough outline of the content and
structure of the presentation is made. In the design layer,
every part of the presentation is designed. In the realization
layer, the design is realized into a full specification of the
presentation in a platform independent way. It is the
presentation display layer that processes the contributions
of these other layers into the final presentation to the user.
A number of expert modules are defined which
assists the generation process. The application expert
provides information about the media used. It can, for
example, say the dimensions of an image file to be used. It
can also provide semantic information about media
objects. The context expert keeps track of the
presentation’s history: what was shown, and how the user
interacted. The user expert conveys information about a
particular user that can be used in tailoring the presentation
for him or her. Remaining information processing that
affects how a presentation is adapted is put in the design
expert.
In the Berlage environment, the functions of the
generation process and knowledge server can be embodied
at least in part by DSSSL code. The knowledge processing
encoded in the Fiets application’s DSSSL code is of a
relatively simple nature, making basic decisions. DSSSL as
a language has been adequate for this. More complex
decision processes may require software outside of DSSSL
to complement what is provided in the Berlage
environment.
In the Berlage environment the control of the
generation of the presentation is handled primarily by
DSSSL processing. To achieve this goal, the amount of
processing done by the http server must be minimized. As
a result, the server is simply used to store old fields and
values from previous URL requests, add new fields and
values from incoming requests, and modify existing fields
with new values from incoming requests. The presentation
state simply conveys these fields and values, and the
DSSSL style sheet has instructions on how to handle this
information.
4.1 Control layer
The control layer determines at each point in a
presentation what goals there are to meet and how best to
meet them. It communicates frequently with the context
expert, which keeps track of what has happened in the
presentation so far. This layer influences the message to be
expressed by a presentation, but does not make any
decisions directly affecting the instantiation of parts of a
particular presentation.
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For example, Figure 4 shows one of the Fiets
interfaces. Here, the user is shown a collection of
Amsterdam buildings about which more detailed
information can be accessed. A thumbnail image of the
front of each building is shown, and each image is the
starting point for a hyperlink leading to more information
about that building. It is a goal of the application that the
user visit each of the buildings. The user is helped in this
task by the changing appearance of this portion of the Fiets
presentation. After the user has visited a building and
returns to this screen, the visited building, and all
previously visited buildings, are shown as faded so that the
user can distinguish them from the buildings that remain to
be presented in detail.
When the user selects a building, Berlage generates
presentation state DSSSL code that when included in the
style sheet for processing causes SMIL code to be
generated that displays detailed information on that
building. In this and all future presentation state DSSSL
code generated, code is introduced for that building
indicating that it has been visited. The control layer style
sheet code for Fiets has instructions that access this code in
determining whether or not to fade the image displayed for
each building in generating the SMIL code representing the
next step in the presentation. The information conveyed by
the presentation state code corresponds with the context
expert of the SRM.
Figure 4: GRiNS Showing a Fiets Presentation
4.2 Content layer
In the SRM, the content layer takes a single goal
passed on by the control layer and refines it to a list of
subgoals. These are in turn transformed into a set of
communicative acts and relations among these acts. This is
carried out by four separate, but communicating
processes—goal refinement, content selection, media
allocation and ordering.
Goal refinement is the determination of the high
level goal of the presentation. In the Berlage environment,
this is considered the task of the author writing a style
sheet. It is not considered here as an automatically
processed task. Other systems, however, may implement
knowledge processing that makes goal decisions at this
level. Since it is not part of Berlage processing, we do not
discuss goal refinement further here.
The content selection process communicates with the
application expert and decides, given some semantic
content, what media objects are appropriate for conveying
it. In the Fiets presentation, when the user has selected a
building the content of the resulting SMIL display must be
determined. This display shows detailed information about
a building, such as close-up images of different parts of it.
This display also conveys more detailed information on
how the building appears on the outside and on the inside.
This associate of semantic descriptions with media
contents in done in Berlage with HyTime. The processing
of this HyTime code corresponds with the SRM’s
application expert.
When the content layer receives a request for
detailed information on a particular building, it determines
what the appropriate semantic content is for displaying this
information. Precisely which buildings to display would be
determined, along with what aspects of each building to
display with it. What media items to use to represent these
is determined elsewhere in the SRM, as is how to display
them.
The media allocation component assigns a particular
medium to a communicative act, producing a media
communicative act. In the Berlage environment, this
corresponds to the selection of particular media objects to
display for given semantic content. In the Fiets example,
the semantic content of detailed information on a
building’s exterior, determined during content selection,
could correspond with particular images of its gable
ornamentation and entranceway. This correspondence is
determined during media allocation.
The instructional code for determining this media
allocation is contained in the content layer portion of the
style sheet. The content layer portion of style sheet the
instructions for determining the media allocation for
displaying given content. In Fiets, such instructions often
Implementing Adaptability in the Standard Reference Model ... Multimedia Modeling 98
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refer to the HyTime-defined structure of the stored
document to determining this media allocation. For
example, HyTime structures in Fiets associate images and
descriptive audio and text of gable ornamentation and
entranceways with individual buildings. This HyTime code
would be accessed by Jade as instructed by the content
layer portion of the DSSSL style sheet to determine the
media files to use for a particular building.
The result of the ordering process is a not necessarily
linear ordering of the media communicative acts.
Navigational hyperlinks, timing relations and spatial layout
are all different ways of expressing ordering relations
among media items in the final SMIL presentation code.
When a communicative act is to be expressed using a
number of media items, these methods can be traded-off
against one another. For example, rather than display many
items together, links can be made among smaller groups of
the items. Also, the sequential playing of items may be a
suitable alternative for laying out items next to each other
at the same time. The choice among these alternatives in
the Fiets application is described in other work [21]. These
different ways of “ordering” the material need to be
captured within the content layer, but may not be finalized
until the design or realization layer [13]. We restrict the
content layer to specifying a structure of links, and leave
the decisions on temporal and spatial layout to other layers.
In the Berlage environment, specifying this link structure
for a given state in the presentation means determining
what to show on the display being generated and what
information should instead be made accessible by links
from this generated display.
4.3 Design Layer
The design layer is split into two communicating
processes: media design and layout design. The processes
of media design and layout design carry on in parallel,
imposing no ordering requirements for which of these
should be finalized first.
The media design component makes decisions on the
“look and feel” of the presentation. It can communicate
with the design expert and make decisions on styles. Style
information can include color, font, background wallpaper
images, and other aspects of the presentation that do not
affect the actual media content or its positioning in space
and time in the presentation.
The layout design component is responsible for the
spatial and temporal arrangement of the media items in the
presentation. In this layer the constraints used for
determining the spatial and temporal layout need to be
generated. These specifications should be internally
consistent before being handed on to the realization layer,
which may, nonetheless, be unable to satisfy them
requiring the generation of a new set of constraints. The
generation of the spatial layout design could occur along
those lines described in the Reference Model for Intelligent
Multimedia Layout [10].
In the Fiets example, one screen display has
thumbnail images for multiple buildings. The arrangement
of thumbnail images on a single screen is a design layer
spatial layout decision. The design layer arranges the
display of the images as from left to right along rows going
from the top to the bottom of the screen. The design layer
states that the images that overflow off the right of one row
wrap around to the left side of the row below it. Another
decision made by the design layer is that if there are too
many buildings to select from, the thumbnails are
distributed among multiple screen displays. Access to each
of these displays is provided with hyperlinks. How these
guidelines map to the exact placement of these images is
determined not here but by the realization layer
4.4 Realization Layer
In this layer the final decision is taken on the media
items to be played in the presentation and their
corresponding spatial and temporal layout. The output of
this layer is the final SMIL code that gets presented to the
user through the presentation display layer, as shown in
Figure 3.
Again, the layer is split into two communicating
processes, paralleling those in the design layer—media
realization and layout realization. The media realization
component ensures that appropriate media items are
chosen. These may already exist, or may be generated
specifically for the task.
The layout realization component calculates the final
temporal and spatial layout of the presentation based on the
constraints specified in the design layer. The duration in
time of an atomic media object’s presentation may be
explicitly encoded in the storage structure or derived from
the content for continuous media. The duration of a portion
of the presentation consisting of multiple media objects can
be calculated on the basis of these objects along with any
other temporal constraints specified in the design layer.
Where links define possible paths among separate
multimedia presentations, there is also a duration perceived
by the user when the link is followed.
In the Fiets example, the exact position of each
building thumbnail and the number of thumbnails on each
screen display is determined. This positioning gives the
building images an even distribution in the screen space.
The exact font size and positioning of the associated text
and hyperlink hotspots are also determined here. The
design layer determines the manner in which thumbnails
are arranged, while the realization layer determines the
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details of this arrangement given the circumstances of a
given presentation.
5.
4.5 Presentation Display Layer
6.
As shown in Figure 3, the presentation display layer
is embodied in the Berlage Environment by GRiNS. which
processes the generated SMIL code for final presentation
to the user.
5 Summary
This paper explored the implementation of
adaptability in environments that are based on the Standard
Reference Model for hypermedia. Adaptability was
discussed in terms of style sheets, particularly as
represented by DSSSL. The Berlage hypermedia
environment and its use of style sheets and existing public
standards, such as HyTime, DSSSL and SMIL, and tools
such as Jade and GRiNS, was described. The integration of
the SRM into the Berlage environment was introduced in
this work. This integration was used to illustrate the issues
presented in this paper regarding the processing of
adaptability and its relationship with the Standard
Reference Model.
6 Acknowledgments
The CMIFed and GRiNS environment was
implemented by Sjoerd Mullender, Jack Jansen and Guido
van Rossum. The artwork and graphic design of the Fiets
application was done by Maja Kuzmanovic. The
development of CMIFed and GRiNS was funded in part by
the European Union ESPRIT Chameleon project.
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