Tidy for Windows - Appending output to existing file

Not sure if this is bug or feature request or usage advice.
I'm using tidy for Windows 25 march 2009 with Windows 7.
How do I get tidy to *append* its output to a file?
The -f switch always seems to overwrite the specified file.
Adding something like:
  >>outfile.txt
to the tidy command line either only has the
last file's output in it or is empty.

-- Thanks, Peter Higginbotham