@Override
	public boolean resolvesToToken(String x, ActionAST node) {
		LabelElementPair anyLabelDef = getAnyLabelDef(x);
		if ( anyLabelDef!=null && anyLabelDef.type==LabelType.TOKEN_LABEL ) return true;
		return false;
	}
$x.y	Attribute: x is surrounding rule, label ref (in any alts)
tool/src/org/antlr/v4/tool/Rule.java