code: public SortedSet<String> fields() {
  		// Optimization: Materialize the fields set
  		if (myFieldSet == null)
  			myFieldSet = new TreeSet<String>(fields.keySet());
  		return myFieldSet;
  	}
docs: Returns a sorted set containing the field names in this schema, sorted by
  their natural ordering.
  
  @return the sorted set of the schema's field names