public void ClearAdjacencyGraph_InvalidatesOldNodes()
        {
            var solver = new PathSolver <Point, PathTestOption>(_testMap);

            // Build a graph based on all three points.
            solver.BuildAdjacencyGraph(new Point[] { _startPt, _reachableEndPt, _unreachableEndPt });

            // Clear the graph, and then rebuild it with just one point.
            solver.ClearAdjacencyGraph();
            solver.BuildAdjacencyGraph(_startPt);

            // Try to find a path to _unreachableEndPt.  PathSolver should complain that it doesn't know about that point.
            PathResult <Point> pathResult = null;

            Assert.ThrowsException <PathfindingException>(() => pathResult = solver.FindPath(_startPt, _unreachableEndPt, PathTestOption.Normal));
        }