The BidirectionalGraph concept refines IncidenceGraph and adds the requirement for efficient access to the in-edges of each vertex.

This concept is separated from IncidenceGraph because for directed graphs efficient access to in-edges typically requires more storage space, and many algorithms do not require access to in-edges.

For undirected graphs this is not an issue, since the InEdges and OutEdges functions are the same, they both return the edges incident to the vertex.

The InEdges() function is required to be constant time. The InDegree and Degree properties functions must be linear in the number of in-edges (for directed graphs) or incident edges (for undirected graphs).