[plt-scheme] The historical role/value of set-car! and set-cdr!

Say that you want a unfold-left that is tail recursive. The only way it 
to use set-cdr!

Chongkai

Grant Rettke wrote:
> Tonight I went through some problems involving set-car! and set-cdr!.
>
> Other than the homework problems, I have yet hear of folks
> recommending using these functions.
>
> I re-read this article:
>
> http://blog.plt-scheme.org/2007/11/getting-rid-of-set-car-and-set-cdr.html
>
> to get a sense of where they fit today.
>
> Historically, though, they must have been there for some really good reason?
>
> What was that reason?
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