Used properties (current behaviour):
     * number (3)
     * pair ((0 . 0))
     * empty-list (’())

   Used properties (after MR 588):
     * number (3)
     * pair ((0 . 0))
     * empty-list (())

   Used properties (Werner’s proposal):
     * number [default: 3]
     * pair [default: (0 . 0)]
     * empty-list [default: ()]

   Used properties (Michael’s proposal):
     * number, default: 3
     * pair, default: (0 . 0)
     * empty-list, default: ()