arc> (require "lib/scanner.arc")
  nil
  arc> (= s (scanner-string "asdf"))
  #3(tagged scanner (#<procedure> . #<procedure>))
  arc> (car s)
  #\a
  arc> (zap cdr s)
  #3(tagged scanner (#<procedure> . #<procedure>))
  arc> (car s)
  #\s