the content identifier string. This must be a valid URI
(Uniform Resource Identifier, see RFC 2396). This string
is required. If a content provider 'implements' some
existent URI scheme (e.g., a content provider offering
access to a file system would implement the 'file' scheme,
and a content provider offering access to IMAP servers
would implement the 'imap' scheme), the content identifier
strings should use that scheme. If on the other hand a
content provider offers access to content for which no
appropriate URI scheme exists, a vendor-specific URL scheme
starting with 'vnd.' must be created (see the Internet Draft
draft-king-vnd-urlscheme-00.txt).
This string will be calculated from the content identifier string
and must be lower-cased(!). It is the 'scheme' the content provider is
registered for. In example, a provider for FTP contents will use
ftp-URL's as content identifiers. The content provider scheme for all
contents provided by that provider will be "ftp".
