/// <summary>
 /// Sets the descriptor used to interperet tag values.
 /// </summary>
 /// <param name="aDescriptor">the descriptor used to interperet tag values</param>
 /// <exception cref="NullReferenceException">if aDescriptor is null</exception>
 public void SetDescriptor(AbstractTagDescriptor aDescriptor)
 {
     if (aDescriptor == null)
     {
         throw new NullReferenceException("Cannot set a null descriptor");
     }
     this.descriptor = aDescriptor;
 }
 /// <summary>
 /// Sets the descriptor used to interperet tag values. 
 /// </summary>
 /// <param name="aDescriptor">the descriptor used to interperet tag values</param>
 /// <exception cref="NullReferenceException">if aDescriptor is null</exception>
 public void SetDescriptor(AbstractTagDescriptor aDescriptor)
 {
     if (aDescriptor == null)
     {
         throw new NullReferenceException("Cannot set a null descriptor");
     }
     this.descriptor = aDescriptor;
 }