@Test(expectedExceptions = IllegalArgumentException.class)
public void testAddDirectedExistingUndirected() {

    EdgeImpl undirected = GraphGenerator.generateSingleUndirectedEdge();
    EdgeStore edgeStore = new EdgeStore();
    edgeStore.add(undirected);
    EdgeImpl directed = new EdgeImpl('5', undirected.target, undirected.source, undirected.type, 1.0, true);
    edgeStore.add(directed);
}