The modifier: idiom is simply a macro, modifier, which accepts a single form. This modifier essentially accepts the argument as an expression and modifies how it operates; in the case of the breakable: modifier, it creates a 'break function which exits the expression.
Another modifier in Anarki is the p-m: modifier (which needs you to (require "lib/defpat.arc")); it transforms any function-defining form into a pattern matching form:
(p-m:mac foo
() ''(the foo)
(x) `'(the foo is ,x))
A while back I also posted a w/collect and w/mcollect macro form, which also works as a modifier: http://arclanguage.com/item?id=4390 . Macros which accept only a body will generally also work as modifiers, without, umm, modification.
