The four primary classes that are provided by the module. These are exported here such that if you want to create your own methods using COOPS, and want to dispatch based on the type of graph, it is possible to do so.
NOTE: <multigraph> is a subclass of <multidigraph>, and <graph> is a subclass of <digraph>. These all inherit from an <abstract-graph> class, but multigraph types and graph types are otherwise not related in any specific hierarchical way. The reason for this is that this model makes it easier to dispatch based on the possibility of multiple edges vs. dispatching on whether or not the edges are directed.