@Override
	public boolean addAll(Collection<? extends T> c) {
		boolean changed = false;
		for (T o : c) {
			T existing = getOrAdd(o);
			if ( existing!=o ) changed=true;
		}
		return changed;
	}
Add {@code o} to set if not there; return existing value if already
there. This method performs the same operation as {@link #add} aside from
the return value.
runtime/Java/src/org/antlr/v4/runtime/misc/Array2DHashSet.java