How come Revtex says acknowledgements are obsolete?

Running revtex4-1 with the preamble:

\documentclass[aps, pre, floatfix, reprint]{revtex4-1}


and the section later:

\begin{acknowledgements}
The authors would like to thank Tex, LaTeX and Friends
for the answer to this question.
\end{acknowledgements}


Gives the warning:

 Class revtex4-1 Warning: Environment acknowledgements is obsolete;


Have I done something wrong here?

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If I could (rep. deficient) I would tag this as 'revtex' but no tags exists at the moment. –  Hooked Aug 16 '10 at 17:30
Tagged it for you. –  Caramdir Aug 16 '10 at 17:32
Very nice example! Since it's not an error, I changed that tag to [warning]. –  Stefan Kottwitz Aug 16 '10 at 17:51

I have never used revtex but the author's guide explicitly states:

Please note the spelling of “acknowledgments.”

This is also the full error message LaTeX produces:

Class revtex4-1 Warning: Environment acknowledgements is obsolete; Use acknowledgments instead.

So replacing acknowledgements by acknowledgments removes the message.

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This is what I get for compiling in a GUI instead of the command-line. Winefish cuts off the warning, so I never got to see the last part (though grammar hints that the semicolon should give more information). Thank you very much for the answer! –  Hooked Aug 16 '10 at 18:05