[plt-scheme] (html-tag () ... )

I was working though the Help Desk, Brief Tour of DrScheme v. 204, XML 
Boxes examples.  (Those {XML,Scheme,Scheme Splice} Boxes are pretty 
slick.)  When I see something like this:

(ul () (li () (em () "Hamlet")) (li () (em () "othello")) (li () (em () 
"Macbeth")))

I'm left curious, what all the () are for?  What magic takes their 
place when they are not () ?

A short answer is fine.

Geoffrey
--
Geoffrey S. Knauth | http://knauth.org/gsk