Instances of this class represent a link between a port and a
relation, between two relations,
or a between two ports. In the first two cases,
the relations are represented by an explicit node in the graph. In the
third case, there is no explicit node representing the relation and
the edge runs directly from one port to the other. Connections are made
and broken by the graph model depending on
which of the above contexts the link is being used in.
Return the relation that this link represents. If the link goes
from a port to a port, then this is the only way to get at the
relation. If the link goes from a vertex to a port, then the
relation will be the container of the vertex.