nE = 2 * nE;
        }

        checkEdges(nE);
    }

    private void checkVertices() {
        checkVertices(DEFAULT_VERTICES);
    }

    private void checkVertices(int nVertices) {
        if (vertices == null) {
            vertices = new TreeSet<>();
        }
    }

    private void checkEdges() {
        int count = DEFAULT_EDGES;

                Set<String> innerSet = new HashSet<>();
                Collections.addAll(innerSet, elements);
                return Collections.checkedSet(innerSet, String.class);
              }
            })
        .named("checkedSet/HashSet")
        .withFeatures(
            SetFeature.GENERAL_PURPOSE,
            CollectionFeature.SERIALIZABLE,
            CollectionFeature.ALLOWS_NULL_VALUES,

                Set<String> innerSet = new HashSet<>();
                Collections.addAll(innerSet, elements);
                return Collections.checkedSet(innerSet, String.class);
              }
            })
        .named("checkedSet/HashSet")
        .withFeatures(
            SetFeature.GENERAL_PURPOSE,
            CollectionFeature.SERIALIZABLE,
            CollectionFeature.ALLOWS_NULL_VALUES,