(mac eif (var (o expr) (o fail) (o ok))
  (with (v (uvar)
         w (uvar)
         c (uvar))
    `(let ,v (join)
       (let ,w (ccc (fn (,c)
                      (dyn err [,c (cons ,v _)] ,expr)))
         (if (caris ,w ,v id)
             (let ,var (cdr ,w) ,fail)
             (let ,var ,w ,ok))))))

(mac onerr (e1 e2)
  (let v (uvar)
    `(eif ,v ,e2 ,e1 ,v)))

(mac safe (expr)
  `(onerr nil ,expr))

(def literal (e)
  (or (in e t nil o apply)
      (in (type e) 'char 'stream)
      (caris e 'lit)
      (string e)))

(def variable (e)
  (if (atom e)
      (no (literal e))
      (id (car e) vmark)))

(def isa (name)
  [begins _ `(lit ,name) id])