[plt-scheme] Good intro to Scheme macros?

Has anyone come across a good introduction to Scheme
macros? I found Shriram Krishnamurthi's "Macros and
their Impact on Language Design" paper and found it to
be a quite good. What other introductory papers are
good starters on Scheme macros? What I'm specifically
after is the rationale for hygienic macros and a bit
of an insight into how the Scheme macro system
evolved. I know that there are a lot of papers on
Scheme macros on readscheme.org but most of them seem
to deal with a specific facet of Scheme macros. Is
there some paper that looks at Scheme macros a bit
more "holistically"? 

TIA

--
Bill Clementson

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