// them to ensure that the validation check happens and has the expected behavior.
    assertNodeRemovedFromGraphErrorMessage(
        assertThrows(IllegalStateException.class, n1AdjacentNodes::size));
    assertNodeRemovedFromGraphErrorMessage(
        assertThrows(IllegalStateException.class, n1Predecessors::size));
    assertNodeRemovedFromGraphErrorMessage(
        assertThrows(IllegalStateException.class, n1Successors::size));