I haven't tried this on 4.2, but in the 4.0 days I used to use an idiom something like
g.remove rast=MASK
g.copy rast=testmask1,MASK
process using testmask1
You might have to toss in some r.mapcalc calls, to actually build testmask1..testmaskn, especially if you want to do things like masking on certain categories, or certain ranges. But still no big deal. And it was usually my preference to keep the various mask files around to remind myself what I had actually masked--saves time debugging and during demos.
Is the water boiling yet?
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Hal Mueller hal@seanet.com
Seattle, Washington (206) 297-9574