[racket] getting complete paths to Racket executables

How do I get the complete path to Racket executables like "raco", in a 
portable way?

More precisely, I would like to get the complete path of the Racket 
executable currently being used.  (From there, it would be good enough 
just to look for an executable of the desired name, like "raco", in the 
same directory.)

"find-system-path" doesn't seem to be doing it for me even on Linux:

$ cd /tmp
$ racket -e "(find-system-path 'exec-file)"
#<path:racket>
$ racket -e "(find-system-path 'run-file)"
#<path:racket>
$ racket -e "(find-system-path 'orig-dir)"
#<path:/tmp/>
$ which racket
/usr/local/racket-5.2.0.7-20111223/bin/racket
$ alias racket
bash: alias: racket: not found

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