I don't know if they are implemented in a specific package (search CTAN), but there's an article in TUGBoat, (TUGboat 22 (2001), 350–352.) that gives an implementation. Here's the article on the author's website:
I suspect that anyone putting this in a package would have used the same names, or at least referred to this article, so searching on CTAN for the command names used there would be a reasonable thing to do.
(Breadcrumb trail: I first learnt of this via this blog post about these commands in iTeX.)
(I'm feeling generous: I just tried the obvious searches on CTAN and nothing came up; so based on that, you should copy the definitions from that article.)