C-c C-y (slime-call-defun):
When the point is inside a defun and C-c C-y is pressed,
(I’ll use  as an indication where the cursor is)
(defun foo () nil)
then “(foo )” will be inserted into the REPL, so that you can write additional arguments and run it.
If FOO was in a different package than the package of the REPL, (package:foo ) or (package::foo ) will be inserted.
This feature is very useful for testing a function you just wrote.
That works not only for defun, but also for defgeneric, defmethod, defmacro, and define-compiler-macro in the same fashion as for defun.
For defvar, defparameter, defconstant: “ *foo*” will be inserted (the cursor is poistioned before the symbol so that you can easily wrap it into a function call).
For defclass: (make-instance ‘class-name )
Inserting calls to frames in the debugger
C-y in SLDB on a frame will insert a call to that frame into the REPL, e.g.,
(/ 0) =>
1: (CCL::INTEGER-/-INTEGER 1 0)
C-y will insert (CCL::INTEGER-/-INTEGER 1 0)