(map eval
     (tree-subst 'loop 'loopform
       (keep [and (caris _ 'mac) (in (cadr _) 'loop 'for 'down)]
             (readfile "arc.arc"))))

(mac loop (withses . body)
  (let w (pair withses)
    `((rfn next ,(map car w) ,@body) ,@(map cadr w))))