@Override
	public IntervalSet or(IntSet a) {
		IntervalSet o = new IntervalSet();
		o.addAll(this);
		o.addAll(a);
		return o;
	}
Compute the set difference between two interval sets. The specific
operation is {@code left - right}. If either of the input sets is
{@code null}, it is treated as though it was an empty set.
runtime/Java/src/org/antlr/v4/runtime/misc/IntervalSet.java