def to_s extended: nil
    # The way to get the superclass' message
    message = super()
    
    # If `extended` is explicitly `false` then just return that
    return message if extended == false
    
    # Otherwise, see if the extended message was explicitly requested,
    # of if we're configured to provide it as well.
    # 
    # Either way, don't add it it's empty.
    # 
    if  (extended || add_extended_message?) &&
        !extended_message.empty?
      message + "\n\n" + extended_message
    else
      message
    end
  end
def to_s extended: nil
    # The way to get the superclass' message
    message = super()
    
    # If `extended` is explicitly `false` then just return that
    return message if extended == false
    
    # Otherwise, see if the extended message was explicitly requested,
    # of if we're configured to provide it as well.
    # 
    # Either way, don't add it it's empty.
    # 
    if  (extended || add_extended_message?) &&
        !extended_message.empty?
      message + "\n\n" + extended_message
    else
      message
    end
  end
Get the message or the extended message.

 @note
   This is a bit weird, having to do with what I can tell about the
   built-in errors and how they handle their message - they have *no*
   instance variables, and seem to rely on `#to_s` to get the message
   out of C-land, however that works.

   {Exception#message} just forwards here, so I overrode that with
   {#message} to just get the *summary/super-message* from this method.

 @param [Boolean?] extended
   Flag to explicitly control summary/super or extended message:

   1.  `nil` - call {#add_extended_message?} to decide (default).
   2.  `false` - return just the *summary/super-message*.
   3.  `true` - always add the *extended message* (unless it's empty).

 @return [String]