button_to HTML options not completely intuitive

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While working on a site in Rails tonight I was adding a button_to that I wanted a JS confirmation requester. Well, based on my albeit beginner knowledge of Rails I thought the following would work:
[source:ruby]
:empty, :confirm => "Really empty your cart?" %>
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As it turned out this wouldn't work, it kept adding "?confirm=Really empty your cart?" to the URL instead of doing a popup. A bit of fiddling later I realized that to work correctly the additional arguments to button_to need to be arrays, e.g.:
[source:ruby]
:empty}, {:confirm => "Really empty your cart?"} %>
[/source]