GMT4: How to make page background transparent?

Hi.
I've looked around but can't find a solution.

I'm using grdimage followed by psbasemap to produce a map. I've created 
a custom colormap and its high-end is white (255/255/255).

I would like to print the white in the map at high elevation but have 
the page background transparent.  Currently the code produces a white 
background outside of the map frame as well as in the interior of the map.

I don't seem to be able to find the right parameter(s) to do this with 
so maybe it's not possible?

J.
-- 
John Helly, University of California, San Diego / San Diego 
Supercomputer Center / Scripps Institution of Oceanography / 760 840 
8660 mobile / stonesteps (Skype) / stonesteps7 (iChat) / 
http://www.sdsc.edu/~hellyj

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