This bug superseds 
        http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13602

There seems to be a general agreement that C-s and C-r are too useful to
be superseded by icomplete, ido and iswitchb(?).

I am proposing C-, and C-. as useful defaults.

For fear of resentment, I am leaving C-s and C-r bindings intact.
Reviewer may delete them at his own discretion.