I consider the SRFI ill-specified considering the commentary at the beginning of
the SRFI and the
informal nature of the specification section.   Compare
>> This SRFI proposes an extension of set! so that it provides similar
functionality as Common Lisps's setf,
>> but in the  "functional (first-class)" manner.
from the Abstract and
>> The special form set! is extended so the first operand can be a procedure
application,
>> and not just a variable.
The examples that follow, however, are all of the shape (set!  (procedure-name
...) ...).
So, where is the syntax specification for this new/extended form? In this day
and age, that's the
minimum we can expect. Is it

  (set!  exp exp)

or

  (set! (p exp ...) exp)

or what?

Also,  if what we see is all there is to it, I would think that a syntax-rules
specification  can be written.
If so, supply it.

-- Matthias