assume().that(graphIsMutable()).isTrue();

    assertThat(graphAsMutableGraph.putEdge(N1, N2)).isTrue();
    assertThat(graphAsMutableGraph.putEdge(N1, N2)).isFalse();
  }

  @Test
  public void putEdge_orderMismatch() {
    assume().that(graphIsMutable()).isTrue();

    EndpointPair<Integer> endpoints = EndpointPair.unordered(N1, N2);
    IllegalArgumentException e =