public XmlDom tag(String tag,String attr,String value){
  List<XmlDom> tags=tags(tag,attr,value);
  if (tags.size() == 0) {
    return null;
  }
 else {
    return tags.get(0);
  }
}

public XmlDom tag ( String tag , String attr , String value ) { List < XmlDom > tags = tags ( tag , attr , value ) ; if ( tags . size ( ) == 0 ) { return null ; } else { return tags . get ( 0 ) ; } }
return a node that represents the first matched tag . if value = = null , node that has the attr are considered a match . a dummy node is returned if none found .
