The VertexDescriptor of the edge's starting point.
The VertexDescriptor of the new edge's end point.
The value to associate with the new edge, if any.
Returns an AddEdgeResult value, containinf the descriptor of the edge, and a flag to let you know if it was newly created (true), or the edge already esisted and the graph type doesn't support parallel edges, so the returned descriptor refers to the pre-existing edge (false).
Adds an edge to the graph.