To avoid more silly mistakes like https://github.com/sweet-js/sweet.js/issues/679#issuecomment-294068431 I've added semicolons to the end of all function expressions defining macros in the code samples.
I think a lot of people who try out Sweet start by copy/pasting from the tutorial. Currently, if they're going to be bitten by [...] and (...) following a syntax, syntaxrec or operator definition. This should ease some frustration.