5.4 Other code in .asd files

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:

• Any informative output (other than warnings and errors, which are the condition system's to dispose of) should be sent to the standard CL stream *standard-output*, so that users can easily control the disposition of output from ASDF operations.