code: public String format(TimeSpan<? super U> duration) {
  
          StringBuilder buffer = new StringBuilder();
  
          try {
              this.print(duration, buffer);
          } catch (IOException ex) {
              throw new AssertionError(ex); // should never happen
          }
  
          return buffer.toString();
  
      }
docs: /*[deutsch]
  <p>Erzeugt eine textuelle Ausgabe der angegebenen Dauer. </p>
  
  @param   duration	duration object
  @return  textual representation of duration
  @throws	IllegalArgumentException if some aspects of duration
  prevents printing (for example too many nanoseconds)