diff --git a/abc/past.nim b/abc/past.nim
index bde5f22..93ab7ba 100644
--- a/abc/past.nim
+++ b/abc/past.nim
@@ -1,24 +1,28 @@
 import ./txstore
 
-func past(store: TxStore, txHash: TxHash, accumulator: var seq[TxHash]) =
+type
+  History* = object
+    transactions*: seq[TxHash]
+
+func past(store: TxStore, txHash: TxHash, history: var History) =
   if transaction =? store[txHash]:
     for (hash, _) in transaction.inputs:
-      if not accumulator.contains hash:
-        accumulator.add(hash)
-        store.past(hash, accumulator)
+      if not history.transactions.contains hash:
+        history.transactions.add(hash)
+        store.past(hash, history)
 
-func past(store: TxStore, ackHash: AckHash, accumulator: var seq[TxHash]) =
+func past(store: TxStore, ackHash: AckHash, history: var History) =
   if ack =? store[ackHash]:
     if previous =? ack.previous:
-      store.past(previous, accumulator)
+      store.past(previous, history)
     for txHash in ack.transactions:
-      if not accumulator.contains txHash:
-        accumulator.add(txHash)
-        store.past(txHash, accumulator)
+      if not history.transactions.contains txHash:
+        history.transactions.add(txHash)
+        store.past(txHash, history)
 
-func past*(store: TxStore, hash: TxHash|AckHash): seq[TxHash] =
+func past*(store: TxStore, hash: TxHash|AckHash): History =
   store.past(hash, result)
 
-func past*(store: TxStore, hashes: varargs[AckHash]): seq[TxHash] =
+func past*(store: TxStore, hashes: varargs[AckHash]): History =
   for hash in hashes:
     store.past(hash, result)
diff --git a/tests/abc/testPast.nim b/tests/abc/testPast.nim
index eef4a3a..0d57e32 100644
--- a/tests/abc/testPast.nim
+++ b/tests/abc/testPast.nim
@@ -27,24 +27,30 @@ suite "Past transactions and acknowledgements":
   test "finds all transactions that precede a transaction":
     var store = TxStore.init(genesis)
     store.add(tx1, tx2, tx3)
-    check store.past(tx1.hash) == [genesis.hash]
-    check store.past(tx2.hash) == [genesis.hash]
-    check store.past(tx3.hash) == [tx1.hash, genesis.hash, tx2.hash]
+    check store.past(tx1.hash).transactions == [genesis.hash]
+    check store.past(tx2.hash).transactions == [genesis.hash]
+    check store.past(tx3.hash).transactions == [
+      tx1.hash, genesis.hash, tx2.hash
+    ]
 
   test "past is empty when transaction cannot be found":
     let store = TxStore.init(genesis)
-    check store.past(tx1.hash) == []
+    check store.past(tx1.hash).transactions == []
 
   test "finds all transactions that precede an acknowledgement":
     var store = TxStore.init(genesis)
     store.add(tx1, tx2, tx3)
     store.add(ack1, ack2, ack3)
-    check store.past(ack1.hash) == [tx1.hash, genesis.hash]
-    check store.past(ack2.hash) == [tx2.hash, genesis.hash]
-    check store.past(ack3.hash) == [tx1.hash, genesis.hash, tx2.hash, tx3.hash]
+    check store.past(ack1.hash).transactions == [tx1.hash, genesis.hash]
+    check store.past(ack2.hash).transactions == [tx2.hash, genesis.hash]
+    check store.past(ack3.hash).transactions == [
+      tx1.hash, genesis.hash, tx2.hash, tx3.hash
+    ]
 
   test "finds all transactions that precede a set of acknowledgements":
     var store = TxStore.init(genesis)
     store.add(tx1, tx2, tx3)
     store.add(ack1, ack2, ack3)
-    check store.past(ack1.hash, ack2.hash) == [tx1.hash, genesis.hash, tx2.hash]
+    check store.past(ack1.hash, ack2.hash).transactions == [
+      tx1.hash, genesis.hash, tx2.hash
+    ]