[plt-scheme] Custom expansion and certificates

I've attached one of the smallest examples producing "access from an 
uncertified context to unexported variable". It's caused by rebuilding 
custom-expanded syntax that contains unexported identifiers.

Okay, it *would* be one of the smallest examples except it contains my 
non-working half-cargo-cult solution. I arrived at it by trying to get 
at the meaning of the solutions in match's pattern parser and for's 
sequence transformer expander. All *I* want to do is rebuild syntax 
lists, perhaps with a couple of macro calls inserted. The attached code 
only tries to rebuild.

I think I have the main idea: put syntax marks on, then transform, then 
have a local certifier put the old certificates on the marked objects. 
My questions:

  - Why syntax-local-introduce first? Does that remove the marks of the 
most recent expansion? Can syntax have only one mark at a time?
  - Why undo the new marks before applying the local certifier?
  - How deep is the local certifier's recursion?
  - When am I losing the certificates? In syntax->list or datum->syntax?
  - Why doesn't the match's pattern expander use syntax-recertify, when 
the sequence transformer expander does?
  - Why use an inactive vs. an active certifier?

I think what would be most helpful is a very small working example. 
Those in match and for are way too complicated to fit in my head.

Is it possible there's a nice syntax/loc sort of solution for simple 
cases like this, or even cases involving deep restructuring?

Neil T
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