Just a draft.

o Defoma-configuration script must use /var/lib/defoma/<package>.d for
  the result of configuration, and must not create or modify any file
  that does not exist under /var/lib/defoma/<package>.d.

o Symbolic link to a directory must not exist under 
  /var/lib/defoma/<package>.d. Symbolic link to a file may exist.

o File or Symbolic link must not exist under /var/lib/defoma/<package>.d
  statically. It means every file and every symbolic link to a file must be
  ones generated by Defoma-confugration script. If the script wants to refer
  a file for configuration, it must be put except under /var/lib/defoma.

o Directory may exist under /var/lib/defoma/<package>.d statically.

o The script must guarantee reconfiguration succeeds when all files and
  symbolic links under /var/lib/defoma/<package>.d are removed.
  It can possibly be realized if the rules above are observed.