abbrev-mode: "SF" -> "SourceForge"?

[Thomas L Roche]

I'm using

GNU Emacs 21.2.1 (i686-pc-cygwin, X toolkit) of 2004-03-22 on cm-test

and getting back into abbrev-mode after a long absence. I notice that
when I have

    ("sf" "SourceForge" nil 0)
;    ("SF" "SourceForge" nil 0)

in my .abbrev_defs, I get

"sf" -> "SourceForge"
"SF" -> "SOURCEFORGE"

By contrast, when I have 

;    ("sf" "SourceForge" nil 0)
    ("SF" "SourceForge" nil 0)

in my .abbrev_defs, I get no mapping, i.e.

"SF" -> "SF"

Can I configure emacs to give the mapping

"SF" -> "SourceForge"

? Apologies if this is a FAQ, but I'm not seeing an answer either in
the Info or the FAQ or googling. Please reply directly to me (as well
as the list) if possible, feel free to forward, and TIA.