TokenNode constructor
      
      TokenNode(
    
    
- String item
Implementation
TokenNode(String item) {
  var arr = item.split('\t');
  if (arr.isNotEmpty) {
    surface = arr[0];
  }
  if (arr.length == 2) {
    features = arr[1].split(',');
  } else {
    features = [];
  }
}