@Override
    public String toString() {
		StringBuilder buf = new StringBuilder();
		buf.append("Rule{name=").append(name);
		if ( args!=null ) buf.append(", args=").append(args);
		if ( retvals!=null ) buf.append(", retvals=").append(retvals);
		buf.append("}");
		return buf.toString();
    }
$x.y	Attribute: x is surrounding rule, label ref (in any alts)
tool/src/org/antlr/v4/tool/Rule.java