public LabelElementPair getAnyLabelDef(String x) {
		List<LabelElementPair> labels = labelDefs.get(x);
		if ( labels!=null ) return labels.get(0);
		return null;
	}
$x.y, x can be surrounding rule, token/rule/label ref. y is visible
 attr in that dictionary.  Can't see args on rule refs.
tool/src/org/antlr/v4/tool/Alternative.java