rs,union-products: Return an empty set when no reaction names are given.

rs.rkt

@@ -51,15 +51,18 @@
     name))
 
 ;;; Returns the union of the product sets of the given reactions in a
-;;; reaction system.
+;;; reaction system.  If no reactions are supplied, returns the empty
+;;; set.
 ;;;
 ;;; This function can be seen as producing the result of the
 ;;; application of the given reactions to a set.  Clearly, it does not
 ;;; check whether the reactions are actually enabled.
 (define (union-products rs as)
-  (apply set-union
-         (for/list ([a as])
-           (reaction-products (hash-ref rs a)))))
+  (if (empty? as)
+      (set)
+      (apply set-union
+             (for/list ([a as])
+               (reaction-products (hash-ref rs a))))))
 
 ;;; Applies a reaction system to a set.
 (define (apply-rs rs s)