Function accepting variadic arguments

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On Wednesday, 20 January 2016 at 12:56:51 UTC, Ali Çehreli wrote:
> And there is another example here:
>
>
> http://ddili.org/ders/d.en/templates_more.html#ix_templates_more.tuple%20template%20parameter
>
> Ali

Thank you!

What's the purpose of "is" in the type checks?

Also, how would I utilize isNumeric from std.traits here, in 
place of checking only for ints?