// AddEdge adds the edge, creating endpoint nodes if necessary.
// Edge is added to adjacency list of from edges.
public void AddEdge(string name1, string name2, string relationship)
{
AddNode(name1); // create the node if it doesn't already exist
GraphNode n1 = nodeDict[name1]; // now fetch a reference to the node
AddNode(name2);
GraphNode n2 = nodeDict[name2];
GraphEdge e = new GraphEdge(n1, n2, relationship);
n1.AddIncidentEdge(e);
}