GenerateUID returns a random string containing timestamp and hostname.

MetaColor is a filename vdir uses for collection color.

MetaDisplayName is a filename vdir uses for collection name.

Collection is a Vdir collection.

DecodeError is an error occured during ical decoding.

Item is an iCalendar item with a unique id.

Tag is a hashtag label in todo summary.

Vdir is a map of all collections and items.