[plt-scheme] Mutable and immutable pairs in PLT Scheme

We teach SICP as a first course, in parallel with a "standard" course  
on algorithms and data structures in Scheme.

I designed the algo&data course with the following constraints in mind:
- the Scheme has to fit SICP. Both courses are first semester courses.
- I wanted a module/library language in order to illustrate ADTs with  
multiple implementations.
- I want to be as close as possible to some standard.

Therefore, I decided to do R6RS without records etc. So I do use  
mutable pairs.

Cheers
Wolf