The idea is that the file is "read in" as if its content was copy-n-pasted.
For example:

"\h|Installation|"
source "INSTALL.cli" // Also used to generate plain text INSTALL.

Unlike include, source can appear anywhere in the file.

For now multiple, non-virtual inheritance is supported. An option class
can now also be declared abstract using the class c = 0 {...}; syntax.
New option, --exclude-base, controls whether base class information is
present in usage and documentation.

New include-path prefixes, c++: and cli:, are now recognized (e.g.,
include <cli:foo>;). Without a prefix, the include declarations is
considered to be c++-include unless the path ends with the .cli
extension.

The cli-included files are loaded and parsed. Currently, only inclusion
relative to the current file is supported. Duplicate inclusions are
detected and ignored based on the absolute filesystem path.

If a file cli-includes another file, then the runtime code is assumed
to come from the included file and is not generated.