[plt-scheme] I can't seem to match on structures in the html module

I haven't yet investigated enough to tell whether this is a problem
with any module or just with html.

Here's a short REPL excerpt.  html-element doesn't seem to be private
to the html library or something, since the definition of f works fine.

  Welcome to MzScheme version 204, Copyright (c) 1995-2003 PLT
  > (require (lib "match.ss"))
  > (require (lib "html.ss" "html"))
  > (define (f x) (html-element? x))
  > (f 3)
  #f
  > (define g (match-lambda [($ html-element attrs) #f]))
  STDIN::151: match: not a defined structure in: html-element

Am I doing something silly wrong?

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