OK, I think I get it:
```
R = Region(3, coordsys)
assert R.__contains__(1,1) # just to show that 1,1 is in the region, nothing else is...
mask = [[0,0,0],[0,0,0],[0,0,0]]
newmask = R.scan(mask)
newmask
[[0,0,0],[0,3,0],[0,0,0]]
```

So if, say, I wanted a mask that selects all pixels in a given region, I'd do:
```
region3_mask = (R.scan(blank_mask)==3)
```