/**
     * Returns the upper boundary of this range, or {@code null} if none.
     */
    @Nullable
    Boundary getUpperBoundary();

    /**
     * Returns the lower boundary of this range, or {@code null} if none.
     */
    @Nullable
    Boundary getLowerBoundary();

    /**
     * Returns a string representation of this version range

                    if (lower == null || upper == null || lower.compareTo(upper) != 0) {
                        restrictions.add(new Restriction(lower, lowerInclusive, upper, upperInclusive));
                    } else if (lowerInclusive && upperInclusive) {
                        restrictions.add(new Restriction(lower, lowerInclusive, upper, upperInclusive));
                    }