subst

[procedure] subst new old seq

(def subst (new old seq)
  (let boundary (+ (- (len seq) (len old)) 1)
    (tostring 
      (forlen i seq
        (if (and (< i boundary) (headmatch old seq i))
            (do (++ i (- (len old) 1))
                (pr new))
            (pr (seq i)))))))