The sample implementation is not thread safe in the sense that I believe
it is call/cc safe.

The combination of (a) general procedures as array getters, (b) mutable
pairs, and (c) general continuations make things a bit tricky for the
sample implementation.  I'll soon propose some small changes to avoid
some crashes/unclear error messages if mutable pairs and general
procedures as array getters are combined in weird ways.

I'm understanding more and more why some Scheme-like languages might
want to make immutable pairs and restricted continuations the default.

Brad