This is SBCL 2.1.0.debian, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.

* (defun my-fn (obj)
    (setf (slot-value obj 's2) 1))    
MY-FN

* (defclass a ()
    ((s1 :initform #'my-fn)
     (s2)))
#<STANDARD-CLASS COMMON-LISP-USER::A>