[gradsusr] "redefining" a value of an expression?

[Christopher Gilroy]

So if I had a var like:

'define var1 = tmp2m/2'

'set gxout grid'
'd var1'


Is there anyway I could change the value of a grid point to say, 0 if it's
a negative number? So the majority of the grid shows positive numbers but
certain sections have negative numbers. I want to be able to just change
them to a set number, 0 for example.
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