[ferret_users] Adding non-consecutive levels containing missing values

["'Fabio B.' via _OAR PMEL Ferret Users" <ferret_users@xxxxxxxx>]

Dear Ferret users,
 
I am on ferret 6.9.6 and I am trying to sum vertical layers which 
contain NaN's.
 
The first part of the question is how to add non-consecutive levels, for 
example I know I can use myvar[k=3:6@sum] for consecutive ones, but how, 
for example, to add levels 2,5, and 7? Do I have to add them 
individually? myvar[k=2]+myvar[k=5]+etc.? Or I can create some kind of 
non-consecutive index such as myvar[k=2/5/7], I can't find this 
information anywhere.
 
The second part, my data contains missing values, when I use @sum NaN 
are ignored and I get the desired result, for example if I have 
3,NaN,2,0 the result is 5. But if instead I use the notation 
myvar[k=2]+myvar[k=5]+etc. then I get NaN in every cell where there is 
at least one NaN. It seems an inconsistent behavior.
 
I saw a few similar questions, but I don't want to convert my NaN to 0 
because they have different meaning in my data.
 
Any help is appreciated. Thank you