Why does amsmath print a warning about a foreign command?

I was wondering what was the cause of the following warning message with the specific \usepackage used below:

Using the following:

Preamble

\documentclass[amssymb,twocolumn,aps]{revtex4}
\usepackage{times,amsmath,latexsym}


Warning message

Package amsmath Warning: Foreign command \atopwithdelims;
(amsmath)                \frac or \genfrac should be used instead
(amsmath)                 on input line 56.


Line 56

$\frac{gcd(m,n)}{n} {n\choose m}$ is an integer for all pairs of integers $n\geq m\geq 1$.

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What is on line 56 of your file? –  Matthew Leingang Jun 1 '11 at 10:48
@Matthew Leingang: It's classified top secret info, I wouldn't be able to disclose that with you, I am sorry. Just joking, it was cool pretending to be a agent there for a bit. It is here below: $\frac{gcd(m,n)}{n} {n\choose m}$ is an integer for all pairs of integers $n\geq m\geq 1$. –  night owl Jun 1 '11 at 12:16
Aha. Replacing n \choose m with \binom{n}{m} will make the warning go away. –  Matthew Leingang Jun 1 '11 at 14:30
@Matt: Thank You, that sure did take care of it flawlessly. –  night owl Jun 1 '11 at 15:49
\atopwithdelims is a TeX math primitive command. If you use amsmath, you don't need \atopwithdelims or \atop, use amsmath macros such as \frac, \binom, \genfrac etc. instead.
If a class or package uses this command, or a command such as \choose which uses it, you would get this warning. I would ignore the warning then, otherwise I would have to redefine the concerned macros of the package or class.
Thanks, that is exactly what it is. Using the \choose command for the binomial coefficients. So you suggest ignoring the warning since I am using the \choose ? –  night owl Jun 1 '11 at 12:13
@night owl: I suggest using \binom{a}{b} instead of {a \choose b}. –  Stefan Kottwitz Jun 1 '11 at 12:30