Possible extensions

<page>

<h1>Possible extensions</h1>
   

<h2>Repeatable components</h2>

<p>
    In a sense, the <x>Dimension</x> and <x>Unit</x> elements operate as model-independent global quantities since they
    may be restated in different files without problems as long as the definitions are identical.
</p>


<p>
    There is probably a case for a user-definable type with similar properties. The canonical application would be 
    in defining the ionic or molecular species present in a simulation.  For example, in specifying that a channel is 
    permeable to Na+ ions, or blocked by TTX, it is necessary to match up the occurrences of these quantities in the 
    channel definition with those in the environment of the cell model in which the channel is used.
</p>
<p>
This can be done with the <x>ExternalQuantity</x> element as in example 5, but that currently requires all references 
to a global component to point to the <i>same</i> component since a component cannot be repeated. This prevents there being
fully independent self contained channel definitions as they would need to share the same species file. It should be 
easily resolved by allowing a flag on type definitions to say that duplicate components are allowed for particular types 
as long as the definitions
are identical.
</p>
 

<h2>Scalar fields</h2>

<p>
    Conceptually, you need a number of things:
    <ul>
        <li>Something like a <x>FieldVariable</x> declaration in a <x>Dynamics</x> analogous to a <x>StateVaraible</x>
        but spatially extended.</li>
        <li>Some notion of geometry</li>
        <li>A mechanism for extracting a value for a <x>FieldVariable</x> at a point for use in a point process model.
        this could involve sampling, interpolation or integration over some kind of kernel</li>
        <li>A mechanism for associating point models with positions in a scalar field</li>
    </ul>
</p>

<p>
    Of these, the geometry and positioning is the hard part. Whether an implementation could actually 
    do something useful with such a model is another question entirely.

</p>

</page>