[ferret_users] How can I check existence of a variable

[Brockmann Patrick <Patrick.Brockmann@xxxxxx>]

Hi all,

In 2005, someone asks how to check if a variable exists.
Read 
http://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2005/msg00828.html

I have tried to use return value but the error message is not trappable

yes? use levitus_climatology.cdf
yes? def sym isexist=`temp,return=dsetnum`
!-> def sym isexist=1
yes? def sym isexist=`abcd,return=dsetnum`
**ERROR: variable unknown or not in data set: ABCD

So is there a way to improve return value mechanism to trap the error 
message
and get a return value set to "ERROR". It would be easy then to test it with
a simple test:

check.jnl

def sym isexist=`($01),return=dsetnum`
if `"($isexist)" eq "ERROR"` then
       say "($01) does not exist"
elif
       say "($01) exists"
fi

Thanks
Patrick