A "custom" List Group implementation. This type of lister uses a completely custom message to show the group, without a need to recursively invoke a lister to list the individual elements. The main difference between this and the base ListGroup is that the interface to the custom message generator is very simple - we can dispense with most of the numerous arguments that the base group message receives, since most of those arguments are there to allow recursive listing of the group list.

This group type is intended mainly for cases where you want to display some sort of collective description of the group, rather than listing its members individually. The whole point of the simple interface is that we don't pass the normal big pile of parameters because we won't be invoking a full sublisting. Since we assume that this group won't itself look like a sublist, we set groupDisplaysSublist to nil by default. This means that our presence in the overall list won't trigger the "long list" format (usually, this uses semicolons instead of commas) in the enclosing list. If your custom group message does indeed look like a sublist (that is, it displays multiple items in a comma-separated list), you might want to change groupDisplaysSublist back to true so that the overall list is shown in the "long" format.

class ListGroupCustom :   ListGroup