The `acronym` packages allows one to easily create and display acronyms, with a simple syntax, like `\ac{usb}`. The first time this is called, is will say “Universal Serial Bus (USB)”, but after that, just “USB”. I found nothing about this in the documentation, but this [article on PDF-strings](http://wiki.contextgarden.net/PDF-strings) seems to offer similar functionality, but the example seems incomplete. How can I define and display acronyms in ConTeXt?