Add test for with-db

test.scm

@@ -199,4 +199,17 @@
     (define get-solution (?- (zebra ? ?water-drinker ?zebra-owner)))
     (define solution (get-solution))
     (assert-equal 'norwegian (assq-ref solution '?water-drinker))
-    (assert-equal 'japanese (assq-ref solution '?zebra-owner))))
+    (assert-equal 'japanese (assq-ref solution '?zebra-owner)))
+  (define-test "with-db"
+    (define my-db '())
+    (define get-solution #f)
+    (define solution #f)
+    (with-db my-db
+             (<- (foo bar)))
+    (set! get-solution (?- (foo ?x)))
+    (set! solution (get-solution))
+    (assert-equal #f (assq-ref solution '?x))
+    (with-db my-db
+      (set! get-solution (?- (foo ?x)))
+      (set! solution (get-solution)))
+    (assert-equal 'bar (assq-ref solution '?x))))