(1) As the sample implementation has never worked with the
unintendedly overly general current spec, I have a good feeling about
this erratum. Apparently, no one has yet exploited what I would like
to restrict.

(2) The key is allowed to be changed. However, the change must be
compatible with the equivalence relation defined by the comparator
used for the mapping. This is needed, for example, to implement
`mapping-set` (as opposed to `mapping-adjoin`).