In case that isn’t clear, the semantics of the configuration is that when searching for a system of a given name, directives are processed in order.
When looking in a directory, if the system is found, the search succeeds, otherwise it continues.
When looking in a tree, if one system is found, the search succeeds. If multiple systems are found, the consequences are unspecified: the search may succeed with any of the found systems, or an error may be raised. ASDF currently returns the first system found, XCVB currently raised an error. If none is found, the search continues.
Exclude statements specify patterns of subdirectories
the systems from which to ignore.
Typically you don’t want to use copies of files kept by such
version control systems as Darcs.
Exclude statements are not propagated to further included or inherited
configuration files or expressions;
instead the defaults are reset around every configuration statement
to the default defaults from
Include statements cause the search to recurse with the path specifications from the file specified.
An inherit-configuration statement cause the search to recurse with the path specifications from the next configuration (see Configurations above).