In this letter I'll rename the specification of mutable-array to avoid confusion, to:
   (mutable-array domain get-proc set-proc)

The "standard" Scheme argument-order for a "set-proc" is to have the new value *last*.
E.g. (vector-set! vec index new-value).  That is why SRFI-17 defines:
   (set! (proc arg ...) value)
as
   ((setter proc) arg ... value)

The setter procedure passed to mutable-array has the order:
   (setter value i1 ... in)
I think it should be:
   (setter i1 ... in value)
That would allow:
   set-proc == (setter get-proc)

Of course the downside of that is that you can't use a "rest" argument to
writer the setter:
   (lambda (value . indexes) ...)
This issue is discussed in SRFI-17.

We can "have the best of both" by changing the get-proc to
take a single vector-valued argument:
   (get-proc (vector i1 ... in))
   (set-proc (vector i1 ... in) value)

One really wants to be using a vector of indexes rather than a list
of indexes.  And preferably a uniform fixnum vector.

Note that SRFI-25 has:
   (array-set! array k ... value)
SRFI-25 also allows array indexes:
   (array-ref array indexes)
   (array-set! array indexes value)
where indexes is a vector or a rank-1 array.
These variants would be the lower-level one, that would call the
mutable-array get-proc and set-proc.  The variable-length array-ref
and array-set! would be convenience methods that call the vector-based ones.
--
	--Per Bothner
xxxxxx@bothner.com   http://per.bothner.com/