This has been discussed before; as CatDancer said, the problem is auto-destructuring of lists. On Anarki, there are two macros, given and givens; they don't require the variables to be parenthesized, but can only have one expression in the body (which can, of course, be a do). given is analogous to with, and givens is analogous to withs. For more info, see http://arclanguage.org/item?id=6759 .
