The proposed syntax for macro-generating macros is
(... ...), which is consistent with existing use.
However, the next level of nested syntax I've seen in use is
(... ... ...) rather than ((... ...)(... ...)). Since it's
more succinct and doesn't lead to an exponential amount of
characters typed per level of nesting, and particularly since
it doesn't lead to ambiguity, I think it's a better convention
than that proposed in the SRFI.
The expansion rule is simple; a sequence of ellipses expands
into a sequence of ellipses that's one shorter, until the
base case is reached.