diff --git a/networks.rkt b/networks.rkt
index 14ab1b2..3d49619 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -106,6 +106,7 @@
                                          (listof (listof (or/c symbol? number?))))]
                [tbf/state-tabulate (->* (tbf/state?) (#:headers boolean?)
                                         (listof (listof (or/c symbol? number?))))]
+               [group-truth-table-by-nai (-> (listof (listof (or/c 0 1))) (listof (listof (listof (or/c 0 1)))))]
                [make-tbn (-> (listof (cons/c variable? tbf/state?)) tbn?)]
                [tbn->network (-> tbn? network?)]
                [make-sbn (-> (listof (cons/c variable? tbf/state?)) sbn?)]
@@ -1298,6 +1299,32 @@
                     (1 0 0)
                     (1 1 0)))))
 
+;;; Given a truth table of a Boolean function, groups the lines by the
+;;; "number of activated inputs"—the number of inputs which are 1 in
+;;; the input vector.
+;;;
+;;; The truth table must not include the header line.
+(define (group-truth-table-by-nai tt)
+  (define sum (((curry foldl) +) 0))
+  (group-by (λ (row) (drop-right row 1))
+            tt
+            (λ (in1 in2) (= (sum in1) (sum in2)))))
+
+(module+ test
+  (test-case "group-truth-table-by-nai"
+    (check-equal? (group-truth-table-by-nai '((0 0 0 1)
+                                              (0 0 1 1)
+                                              (0 1 0 0)
+                                              (0 1 1 1)
+                                              (1 0 0 0)
+                                              (1 0 1 0)
+                                              (1 1 0 1)
+                                              (1 1 1 0)))
+                  '(((0 0 0 1))
+                    ((0 0 1 1) (0 1 0 0) (1 0 0 0))
+                    ((0 1 1 1) (1 0 1 0) (1 1 0 1))
+                    ((1 1 1 0))))))
+
 ;;; A TBN is a network form mapping variables to tbf/state.
 ;;;
 ;;; The tbf/state must only reference variables appearing in the