[gradsusr] define spanning over multiple expressions

[Davide Sacchetti]

Dears, I'd like to define a variable from more than one expression, but
    I don't know how.

I mean: I have different files with temperature (on the same grid), I
could compare temp.1 vs temp.2 but I'd like to compare something like
temp(e=1) vs temp(e=2) without writing a new ctl with ens dimension: 
I'd prefere to do it on the fly, as defining a variable, let's say
tempdef that links to temp.1 for e=1 and temp.2 for e=2, but I don't
know how to do it.
Any ideas?

Bye
Davide