implied contracts in the mapping functions?

[Scott E. Fahlman <Fahlman@CMU-CS-C.ARPA>]

I think that "it is an error" to destructively mess around with a list
after it has been fed to MAPCAR or even to DOLIST.  The fact that most
implementations would do this in a certain way and the user can guess
what that way is should not matter.  If we let people twiddle hidden
state and count on the results, there will be no safe optimizations
left, and all Common Lisp implementations will slow down by a
considerable factor.

-- Scott