[tex-eplain] I need help with index proofing

[John Frampton]

Perhaps I am missing something.  If I make the file foo.tex

-----------------------------------------------------------
\input eplain
\indexproofingtrue
\idx{Jack} and \idx{Jill} went up the hill.
\bye
-----------------------------------------------------------

and then do

    tex foo

the index file foo.idx is created containing what seems like the 
correct information, but no proofing information shows up on the 
page where Jack and Jill appear.

It is my understanding from the documentation that it should.
What's wrong?

Thanks,
John Frampton