public SortedSet<String> fields(){
  if (myFieldSet == null)   myFieldSet=new TreeSet<String>(fields.keySet());
  return myFieldSet;
}

public SortedSet < String > fields ( ) { if ( myFieldSet == null ) myFieldSet = new TreeSet < String > ( fields . keySet ( ) ) ; return myFieldSet ; }
returns a sorted set containing the field names in this schema , sorted by their natural ordering .
