[plt-scheme] macros that need to eval

Here's a riddle: is there a way to write a macro that needs to
evaluate one of its arguments at expansion time-- without using eval?

For example, can you write a macro arity-thunk such that

    (arity-thunk 4 (newline))

expands into

    (lambda (x1 x2 x3 x4) (newline))

?  (The parameters don't have to be named that way; they could be
generated temporaries.)

--dougo at ccs.neu.edu