How do you calculate the length of a singly-linked list that may be cyclic? CLHS includes a nice implementation: https://clhs.lisp.se/Body/f_list_l.htm
miniblog.
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Common Lisp has a function change-class that lets you change the class of instance (like reassigning __class__ in Python). http://clhs.lisp.se/Body/f_chg_cl.htm
I love the note at the end: "semantic difficulties" if you call any methods whilst the class changes!