(define-syntax :gen
  (syntax-rules (index)
    [(_ cc var (index i) expr)
     (:parallel cc (:gen var expr) (:integers i))]
    [(_ cc var expr)
     (:do cc
          (let ([gen expr]))
          ([var (gen)])
          (not (eof-object? var))
          (let ())
          #t
          [(gen)])]))