Called when some process requests clipboard data while this client owns the clipboard. The requested format is passed to the method, and the result should be a string matching the requested format, or #f if the request cannot be fulfilled. (Only data format names in the client's list will be passed to this method; see also add-type.)
The format string is typically four capital letters. (On the Macinotsh, only four characters for format are ever used.) For example, "TEXT" is the name of the simple text format. New format names can be used to communicate application- and platform-specific data formats.