@Override
	public boolean removeAll(Collection<?> c) {
		boolean changed = false;
		for (Object o : c) {
			changed |= removeFast(asElementType(o));
		}

		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