function insertsign(clip mainclip, clip overlayclip, int startframe, int "endframe") {
endframe = default(endframe,startframe+overlayclip.framecount)
number = (mainclip.framecount==endframe) ? -1 : 0
endframe = default(endframe,startframe+overlayclip.framecount+number)
begin = (startframe == 1) ? mainclip.trim(0,-1) : mainclip.trim(0,startframe-1)
middle = mainclip.trim(startframe,endframe)
end = mainclip.trim(endframe+1,0) # BUG: setting endframe=0 doesn't do what you think it does.
middleoverlay = Overlay(middle, overlayclip, mask=overlayclip.showalpha())
begin = (startframe == 0) ? BlankClip(mainclip, length=0) : begin
end = (endframe == mainclip.framecount+number) ? BlankClip(mainclip, length=0) : end
return begin ++ middleoverlay ++ end
}