From the Avisynth 2.05 documents zip describing the
use of Trim()
Trim(clip, int first_frame, int last_frame)
A negative value of last_frame returns the frames from
first_frame to first_frame + (- last_frame-1).
Trim(100,-100) # is the same as Trim(100,199)
When I try Trim(100,-100) I don't get the same
results as Trim(100,199) but instead get the same as
Trim(100,100), just one frame of video.