vnames

A user got bitten by using a vname which could not be used in RPN.

The manual pages warn for this, but apparently nobody noticed the problem 
when the user asked for help.  I admit that I did not immediately spot it 
and needed to do some debugging before being able to solve the problem.
(see the rrd-users mailing list)

Maybe rrdtool should refuse to use, or at least warn about, vnames which are 
also existing operators in RPN?

cheers,
Alex
P.S.
No, I cannot send a patch due to circumstances.