Transpose a complete MIDI file and save it to a new MIDI file (in batch mode)

Hy,

I am a newbie in MIDI events.

I see a transpose proc in midish manual. I would like to transpose a
complete MIDI song (every notes) one or more halftone down or up and save it
to a new MIDI file.

Please, could you give me some advice on realizing this task with midish ?

-- eleandar