This is a simple implementation of a multimap, based on "Data.Map".
['v1.1'] '!' had its arguments flipped. Fixed. Also added 'fromMap'.
['v1.2'] Added "Data.SetMap", renamed 'Multimap' to "Data.MultiMap".
Fixed the type of 'delete'. Derive instances for 'Data' and 'Typeable'.
['v1.2.1'] Fixed typos in the documentation.