Given SYSTEM, the file name of a system generation, return two values: the list of channels SYSTEM is built from, and its configuration file. If that information is missing, return the empty list (for channels) and possibly #false (for the configuration file).

Return a gexp that evaluates to a list of POSIX shell statements defining VARIABLES, a list of variable name/value pairs, as shell variables (not environment variables). The returned code ensures variable values are properly quoted.

Return a service that extends TARGET with VALUE. This works by creating a singleton service type NAME, of which the returned service is an instance.