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Files containing defsystem forms
are regular Lisp files that are executed by load.
Consequently, you can put whatever Lisp code you like into these files
(e.g., code that examines the compile-time environment
and adds appropriate features to *features*).
However, some conventions should be followed,
so that users can control certain details of execution
of the Lisp in .asd files:
*standard-output*,
so that users can easily control the disposition
of output from ASDF operations.