Syntax for duplicating template facts in CLIPS

Question

In the official examples there is a dilemma1 file that contains the following snippet

(defrule MAIN::move-alone
  ?node <- (status (search-depth ?num)
                   (farmer-location ?fs))
  (opposite-of ?fs ?ns)
  =>
  (duplicate ?node (search-depth =(+ 1 ?num))
                   (parent ?node)
                   (farmer-location ?ns)
                   (last-move alone)))

I noticed that the duplicate action uses the =(+ 1 ?num) syntax which I've previously seen only as a predicate/pattern for the left side within a defrule even though it appears on the right in this case.

Section 12.9.4 of the latest CLIPS Reference Manual describes the syntax of duplicate as follows:

(duplicate <fact-specifier> <RHS-slot>*)

The <RHS-slot> rule in Appendix G however does cover function calls and nested expression but it doesn't cover the = prefix. As such I'd like to know why this syntax is valid, what is means and whether the BNF in the manual is just not correct for this case.

Answer 1

It's vestigial syntax from a quarter century ago for compatibility. The syntax for CLIPS was based on ART, a LISP-based tool. ART support lists within lists, so the command

(assert (number (+ 3 4)))

would create the fact

(number (+ 3 4))

In order to perform a function call, you had to use the syntax

(assert (number =(+ 3 4)))

to create the fact

(number 7)

We eventually dropped the requirement for using = to signify a function call in the actions of the rule, but went ahead and kept it for the conditions of the rule because there was also the : constraint which invoked a function call as well as other constraints (&, |, and ~) which had no meaning when used in the actions of a rule.