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services needed for its enactment is called a Unit
of Learning (UoL).
Sequencing adaptation in IMS-LD can be
done through the use of conditions and properties.
Properties have a name and an identifier, a type, a
value and some restrictions on the possible values
they can hold. Conditions are evaluated against
property values. Conditions are evaluated every
time the value of a property changes. When a con-
dition is satisfied, some previously defined action
is executed. With these mechanisms, properties
can be used to store specific information about
the learner and used to adapt the current UoL to
particular needs of a learner or a group by means
of conditions and actions. There is one additional
set of elements of IMS-LD that are important
for sequencing. Global elements are defined to
be included in XHTML resources. They allow
properties to be accessed and modified by the
users through forms in XHTML documents. This
is the only mechanism for users to modify the
properties in a UoL. As a consequence, all other
sequencing adaptation decisions must be included
in the UoL at design time.
activity to be completed, then this activity stays
completed in the run (IMS, 2003c, page 28).
This represents a problem in scenarios when
visiting again the same activity makes sense
from a pedagogical point of view. For example,
a lifelong learner might be interested in revisiting
exercises of a training course performed in the past.
The solution to this problem requires a coupling
between the imsmanifest.xml and the learning
resources. In particular, the <imsld:class> property
of the <imsld:show> element must be used. These
<imsld:class> elements are directly related with
the attribute of the same name in a DIV element
in a XHTML document. A DIV element does not
have any function by itself: it is used as a container
for other elements. It is possible to apply effects
jointly to a group of elements contained in a DIV.
These effects may vary: CSS styles, alignment,
etc. In a DIV, the goal of the class attribute is to
associate the content elements with a style type
defined in the CSS page.
IMS-LD uses the class attributes at the DIV
elements for something different: using show
and hide actions, DIVs can be used to reveal
or conceal different parts of a resource. This is
illustrated in XML excerpts 1 and 2, the former
showing a fragment of the structure of a sample
resource document and the latter depicting the
corresponding condition. If the condition is true,
the IMS-LD player hides all those elements inside
the DIV with a class value of 'exercise-medium'
or 'exercise-hard', and show all the elements
inside the DIV with a class value of 'exercise-
easy'. Thus, the user sees on his/her screen only
the exercise with the easier figure.
The elements <imsld:show> and <imsld:hide>
are used for implementing the behavior defined
by SG in IMS-LD, showing some activities and
hiding others depending on some conditions.
Resources have to be showed to and hidden from
the user, with regard to their relation to an
<sg:exercise>. The only option is to generate one
single resource master file; the content of all the
The Activities' Challenge
As explained in (Gutierrez-Santos et al., 2008),
one important limitation of IMS-LD in terms of
generalised sequencing is how challenging it is to
return to the same exercise to do it again. Unlike
other systems, where exercises are activities that
can be performed over and over again, in IMS-LD
all activities are supposed to be performed only
once. When a student has performed and completed
an activity, it is assumed by the learning design
that its goals have been achieved, so there is no
point in repeating it. This philosophy is specifi-
cally stated in the specification with these words:
... a control must be available in the user-interface
to set the activity status to 'completed'. A user can
do this once (no undo). Once he/she indicated the
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