On this page:
transcript-on
transcript-off

 (require mzlib/transcr)

The transcript-on and transcript-off procedures of mzscheme always raise exn:fail:unsupported. The mzlib/transcr library provides working versions of transcript-on and transcript-off.

(transcript-on filename)  any
  filename : any/c
(transcript-off)  any
Starts/stops recording a transcript at filename.