How preserve existing alpha with -separate?

"Eric J. Chadwick" <[email protected]>
Newsgroups gmane.comp.video.image-magick.user
Message-ID <[email protected]> 
My input is a 32bpp TGA saved from Photoshop CS3, and I need IM to
invert the blue channel, then swap the blue and green channels, then
output a 32bpp TGA again. 
 
It all works fine, except I can't get IM to preserve the pre-existing
alpha channel data, instead it fills the alpha channel with white. 
 
convert in.tga -type TrueColorMatte -channel blue -negate +channel
-separate -channel all -swap 1,2 -combine out.tga
 
The argument "-type truecolormatte" only seems to create a solid-white
output alpha, it doesn't seem to preserve the existing alpha. I also
tried using "-colorspace transparent" or "alpha on" but those didn't
help either.
 
Any ideas?
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