Nitpicking: that's not ssexpansion. Ssexpansion is short for symbol-syntax expansion, in which the occurrence of special characters in symbols tells the arc compiler to transform them, things like foo.bar -> (foo bar), foo!bar -> (foo 'bar). The brace transformation is actually accomplished via changing the reader table of the underlying scheme, and so is translated at read-time rather than compile-time.
This is important if, eg, you're writing scheme code to integrate into the arc runtime in some fashion; normally, in plt scheme, braces [] and parentheses () are interchangeable; but arc changes the way [] is parsed, so you basically have to avoid using them in scheme code. Whereas ssyntax is parsed by the arc compiler, not the reader, so you don't need to worry about it.
