How to "single-step" in slime+emacs

Hello,

(using :editor emacs :editor-package slime :lisp sbcl :platform
linux).
(Consider :following-code)


(declaim (optimize (safety 3)) (optimize (debug 3)) (optimize (speed
0)))

(defun callee (arg1 arg2 arg3)
(let ((combo (+ arg1 arg2 arg3)))
(break "first")
(sin (* pi combo))
(break "second ~f" combo)))

(defun caller ()
(callee 1 2 3))

(sheepishly), 　How can I step from caller into callee? I've been
trying to figure it out and read up on it, but to no avail.

（The break statements are from other experiments that eliminated some
other silly questions)

Thank you,

Mirko
.