compare to nil in clips

Question

How do I compare to nil in clips ?
I have tried

(defrule update-time                                                        
    ?cur-time <- (current-time ?time)                                           
    (order (event-time ?new-time-o&:(neq ?new-time-o nil)))     

I have also tried

(neq ?new-time-o "nil")

none of it seems to work, I get no activation.

Answer 1

You haven't provided enough information to answer your question. Given the appropriate data, this rule will activate:

CLIPS> (deftemplate order (slot event-time))
CLIPS> 
(defrule update-time                                                        
   ?cur-time <- (current-time ?time)                                           
   (order (event-time ?new-time-o&:(neq ?new-time-o nil)))    
   =>)
CLIPS> (assert (order (event-time (time))))
<Fact-1>
CLIPS> (assert (current-time (time)))
<Fact-2>
CLIPS> (agenda)
0      update-time: f-2,f-1
For a total of 1 activation.
CLIPS>