datalog: magic existence check (bb-adornment) regression test (228/228)

lib/datalog/tests/magic.sx

@@ -211,6 +211,23 @@
             (= (len semi) (len magic))))
         true)
 
+      ;; All-bound query (existence check) generates an "bb"
+      ;; adornment chain. Verifies the rewriter walks multiple
+      ;; (rel, adn) pairs through the worklist.
+      (dl-mt-test! "magic existence check via bb"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c). parent(c, d).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (let
+            ((found (dl-magic-query
+                      db (list (quote ancestor) (quote a) (quote c))))
+              (missing (dl-magic-query
+                         db (list (quote ancestor) (quote a) (quote z)))))
+            (and (= (len found) 1) (= (len missing) 0))))
+        true)
+
       ;; Magic over a rule whose body contains an aggregate.
       ;; The rewriter passes aggregate body lits through unchanged
       ;; (no propagation generated for them), so semi-naive's count