The stable branch has this bug as well.
I'd compare how x-set-car! is implemented in the two versions. Current HEAD: https://github.com/arclanguage/anarki/blob/6f3fecdaa46e25b34...
stable: https://github.com/arclanguage/anarki/blob/stable/ac.scm#L12...
I suspect Racket's changed something in unsafe-set-mcar! during the move to Chez Scheme.
(Sorry I just saw this.)
Just after that, Racket introduced an easier way to fix this, `unsafe-set-immutable-car!`, specifically as a stable way to mutate the so-called immutable cons cells going forward. :) We should probably move to that.
I've opened an issue for it: https://github.com/arclanguage/anarki/issues/218
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