FWIW, "first" and "rest" are literally aliases for "car" and "cdr" (and there's a bunch more, see [0]), so it may be that the code you're working with isn't following a good style. The general style rule for more modern code is that you use "first"/"rest" if you're dealing with regular lists, and "car"/"cdr" if you're doing something more peculiar with cons cells - though it's not universally followed, and I guess at this point "car"/"cdr" & friends are too established in the history of Lisp.

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[0] - http://clhs.lisp.se/Body/f_firstc.htm