diff --git a/collects/datalog/ast.rkt b/collects/datalog/ast.rkt
index 99311e889d..84cafa9694 100644
--- a/collects/datalog/ast.rkt
+++ b/collects/datalog/ast.rkt
@@ -11,8 +11,16 @@
                 (or/c exact-nonnegative-integer? #f)
                 (or/c exact-positive-integer? #f))))
 
-(define datum/c (or/c string? symbol?))
-(define datum-equal? equal?)
+(define-struct predicate-sym (srcloc sym) #:prefab)
+(define datum/c (or/c string? symbol? predicate-sym?))
+(define (datum-equal? x y)
+  (match* (x y)
+    [((predicate-sym _ x) y)
+     (datum-equal? x y)]
+    [(x (predicate-sym _ y))
+     (datum-equal? x y)]
+    [(x y)
+     (equal? x y)]))
 
 (define-struct variable (srcloc sym) #:prefab)  
 (define (variable-equal? v1 v2)
@@ -21,8 +29,6 @@
 (define (constant-equal? v1 v2)
   (equal? (constant-value v1) (constant-value v2)))
 
-(define-struct predicate-sym (srcloc sym) #:prefab)
-
 (define term/c (or/c variable? constant?))
 (define (term-equal? t1 t2)
   (cond
diff --git a/collects/datalog/pretty.rkt b/collects/datalog/pretty.rkt
index 697684e346..255f0736a7 100644
--- a/collects/datalog/pretty.rkt
+++ b/collects/datalog/pretty.rkt
@@ -5,12 +5,16 @@
          "private/pprint.rkt"
          "ast.rkt")
 
-(define (format-datum s)
-  (cond
-    [(symbol? s)
-     (text (symbol->string s))]
-    [else
-     (text (format "~S" s))]))
+(define format-datum
+  (match-lambda
+   [(predicate-sym _ s)
+    (format-datum s)]
+   [(? symbol? s)
+    (text (symbol->string s))]
+   [(? string? s)
+    (text (format "~S" s))]
+   [(? number? s)
+    (text (format "~S" s))]))
 (define (format-variable v)
   (format-datum (variable-sym v)))
 (define (format-constant c)