/**
     * Constructs a new set containing the elements in the specified collection. 
     * The {@code HashMap} is created with default load factor (0.75) and an initial
     * capacity sufficient to contain the elements in the specified collection.
     */
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }