z-level problem

[Gavr]

Hello everyone!

I have a problem with new arrays in GrADS:
Where I create new array within old one - as a result I cannot display
it in z-direction. Its possible for arrays described in control-file
only.
For example:
if "V" is described in control-file, that I can display in on the next
z-surface:
#d V(z+1)
but if:
#set z 1 26
#tmp=v*v/2
#set z 1
#d tmp              [works]
but
#d tmp(z+1)         [doesn't work - it returns "entire grid undefined"]

It happens on z-levels only, xy levels are Ok.
Does anybody know what's that?
Thank you!