Crop / clip a SpatialPixels* object

Hi,

This should be simple, but I simply can't find an "elegant" solution.

I have a SpatialPixelsDataFrame that I would like to clip (crop) to a
smaller object. For example,

data(meuse.grid)
m = SpatialPixelsDataFrame(points = meuse.grid[c("x", "y")], data = meuse.grid)

bbox(m)

min    max
x 178440 181560
y 329600 333760

I would like to constrain m, to all y > 331000 and x >180000. What is
the most efficient way to do this (and have the other functionality
e.g bbox and plotting) reflect this reduced data set.

Mark