Cool idea with splitting function arguments into separate lines. It's a bit verbose, but avoids some of Python's old problems regarding what to do about inline lambdas.

But on the down side, now macros have to do string formatting (like adding indents and newlines) to produce output that is legible code, for example the "when" macro:

  defmacro when input
         fn blocks
            ap "if"
               ++ z:indent-before 3
                                  car blocks
                  ++ "\n"
                     ++ z:indent 3
                                 car cdr blocks
                        ++ "\n"
                           z:indent 3
                                    "unit"
            z:blocks input

That's one place where all languages that replace s-expressions with indentation seem to stumble equally: nested parens may look funny, but metaprogramming in lisps is delightfully easy because everything is a list and whitespace doesn't matter, so it's easy to produce and consume at compile time.