There are two parts to this system:
A collapsible <div/>, created using bs_collapse()
At least one button (<button/>) or link (<a/>)
to which the id of the collapsible <div/> is attached,
using bs_attach_collapse()
The verb attach is used to signify that we are attaching the
id of our collapsible
<div/> to the tag in question (a button or a link).
Note that you can attach the id of a collapsible
<div/> to more than one button or link.
It is your responsibility to ensure that id is unique
among HTML elements in your page. If you have non-unique id's,
strange things may happen to your page.