Presentation Specifications
The mechanism of the anchor identifiers can be combined with the component
specification mechanism to provide a way of specifying the endpoints of
a link.
In the Dexter model, this is captured by an entity called a specifier
which consists of a component specification, an anchor id, and two
additional fields: a direction and a presentation specification.
A specifier specifies a component and an anchor "point" within a component
that can serve as the endpoint of a link.
The direction encodes whether the specified endpoint is to be considered
a source of a link, a destination of a link, both a source and a destination,
or neither a source nor a destination. (These are encoded by direction
valued of FROM, TO, BIDIRECT and NONE respectively.)
The presentation specification is a primitive value that forms part of the
interface between the storage layer and the runtime layer.
It contains information specifying how to present the anchor(s) in the
displayed component.
For displaying a component, the runtime model uses an instantiator
function which also uses a presentation specification, containing information
specifying how the component is to be "presented" by the system.
Also these presentation specifications are primitive in the model.