The command evaluates a given expression for each neuron and set visible the neurons which return the value TRUE. Typically the fields in the expression are the variables of the neurons.
Example (ex7.16): The following commands hide neurons without any data records.
...
NDA> somtr -d predata -sout s1 -l 5
NDA> somcl -d predata -s s1 -cout cld1
NDA> clstat -d boston -c cld1 -dout statdata -hits
...
NDA> mkgrp grp1 -s s1
NDA> setgdat grp1 -d statdata
...
NDA> nvsbl grp1 -expr 'statdata.hits' > 0; -v
{{10} > {0}} => TRUE}
{{0} > {0}} => FALSE}
...
