drop anonymous variables when considering if negation is safe

src/Datalog/Rule.elm

@@ -89,6 +89,7 @@ isNegationSafe (Rule _ body_) =
         |> (\{ positive, negative } ->
                 negative
                     |> Set.dropIf (Set.memberOf positive)
+                    |> Set.dropIf ((==) Term.Anonymous)
                     |> Set.isEmpty
            )
 

tests/Datalog/RuleTests.elm

@@ -38,6 +38,14 @@ ruleTest =
                     , negative (atom "reachable" [ variable "a", variable "b" ])
                     ]
                     |> Expect.ok
+        , test "negative terms which are introduced outside the head are still allowed if they appear in positive form" <|
+            \_ ->
+                rule
+                    (atom "peopleWithoutEmails" [ variable "Name" ])
+                    [ positive (atom "people" [ variable "Id", variable "Name" ])
+                    , negative (atom "peopleToEmails" [ variable "Id", anonymous ])
+                    ]
+                    |> Expect.ok
         , test "negative terms are not allowed if they don't also appear in a positive form" <|
             \_ ->
                 rule