Overlay of different but overlapping regions

Dear all,

I want to produce an orthographic projection of Antarctica with a grid
superimposed, but I want the grid to go only from 90 deg East to 180
deg East. The commands

pscoast -R0/360/-90/-55 -JG0/-90/6i -Df -Gblue -K > test.ps
psbasemap -R0/360/-90/-55 -JG0/-90/6i -B15g15 -K -O >> test.ps

correctly produce a "complete" grid around the Pole, but if I try

pscoast -R0/360/-90/-55 -JG0/-90/6i -Df -Gblue -K > test.ps
psbasemap -R90/180/-90/-55 -JG0/-90/6i -B15g15 -K -O >> test.ps

the grid is drawn in another position and with a different dimension
(too big). I understand this is to be expected, and the two commands
are independent: psbasemap does not know that pscoast was called
before and with which parameter.

But, how do I obtain the result I'm looking for?

Thanks!

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