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<h1>What is LSD?</h1>
<p>
LSD (just a name, not an acronym) is a semi-structured notation which we use
in empirical modelling for making an initial analysis of a domain or 
application. As an example, some fragments of the LSD for the vehicle
cruise control model are given below.
</p>

<p>
While LSD is a simple notation, it relates to a fundamental, primitive stage in
the modeller's conception of the real world problem or subject. As a notation
it is still in a process of evolution. It has been usefully applied to many
domains in its present (fairly stable) form, but there are no doubt further
refinements and revisions to be made in the light of further experience.
Suggestions of the problems and limitations of its present form, and for 
improvements, are welcome.
</p>

<p>
The main idea is to record what the modeller regards as the relevant features
(observables) of the subject to be modelled. In the course of identifying these
observables and from experience of the subject domain, the modeller also
needs to identify the 'sources of change', the so-called state-changing
'agents' of the system. The observables are then grouped
around the agents to which they are related. They are classified according
to the way in which they are related to an agent as follows:
</p>

<ul>

<li>
The agent may 'own' them (in the sense that without the agent the observable
could not exist - e.g. the acceleration of a vehicle). These are called
<strong>state</strong> variables.
</li>

<li>
The agent may be able to respond to them (<strong>oracles</strong>).
</li>

<li>
The agent may be able to change them (<strong>handles</strong>).
</li>
</ul>

<p>
These categories are not exclusive (e.g. the variable engineStts is both a
handle and an oracle to the agent driver in the fragment below).
</p>

<ul>
<li>
Where the modeller can perceive (or knows from some theory for
example) that there are some dependencies holding between variables, then the
dependent variable is called a <strong>derivate</strong>, and the dependency relation is
included in the LSD script.
</li>

<li>
Finally where an agent is perceived (or otherwise known) to have the
conditional ability to make changes to variables these privileges are
expressed in guarded sequences called <strong>protocols</strong>.
</li>
</ul>

<p>
Thus an LSD analysis is intended to capture the basic features of the 
initial conceptualisation (or mental picture) of a subject by a modeller. It
is therefore initially a provisional, subjective, personal analysis. It will
typically represent one or more viewpoints (via the agents identified) on the
subject. An LSD script does not have a formal semantics and is not executable.
It embraces a whole range of possible behaviours.
</p>

<p>
A given LSD analysis may be viewed both as a description of a system (from an
external observer's viewpoint), and as an informal program specification. To 
derive an executable program from an LSD analysis we need to make further 
assumptions in order to prescribe particular behaviours for the sake of
display and animation. Such further assumptions can be made in the context
of the ADM (Abstract Definitive Machine); sometimes they can also be made
directly in the notations Donald or SCOUT used for visualisation and
animation.
</p>

<h2>Example fragments of an LSD description:</h2>

<p>
The following fragments of the LSD analysis for the vehicle cruise control
illustrate all the above features and distinctions made in LSD as it is 
currently used. A complete LSD analysis for the cruise control can viewed in
~wmb/public/demos/cruise/cruise.lsd.
</p>

<pre>
agent vehicle {
.......
state           actSpeed : analog       /* actual speed */
                accel    :  analog      /* acceleration */
                windF    :  analog      /* wind resistance force */
 
......
 
oracle          brakePos
 
derivate        windF = windK * sq(actSpeed)
 
                .....
                accel = (tracF - brakF - gradF - rollF - windF) / mass
                actSpeed = integ_wrt_time(accel,0)
 
.......
 
agent driver {
 
 
handle          engineStts, cruiseStts, cruiseSpeed
 
oracle          engineStts, cruiseStts, ...
 
derivate        brakePos = user_input(brakePos_Type)
 
                ....
 
protocol        engineStts == esOff -> engineStts = esOn      /* engine on */
 
                ....
cruiseStts !== csOff -> cruiseStts = csOff    /* switch off cruise control */
</pre>

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