diff --git a/opqueue.go b/opqueue.go
index 1d6dd66..46b66ae 100644
--- a/opqueue.go
+++ b/opqueue.go
@@ -36,6 +36,8 @@ type OpQueue struct {
 	q       *list.List
 	entries map[ID]*OpSet
 	closed  bool
+
+	reducer func(ops []*Op, op *Op) []*Op
 }
 
 // NewOpQueue create a new OpQueue.
@@ -45,11 +47,17 @@ func NewOpQueue(depth, width int) *OpQueue {
 		width:   width,
 		q:       list.New(),
 		entries: map[ID]*OpSet{},
+
+		reducer: appendOp,
 	}
 	q.cond.L = &q.mu
 	return &q
 }
 
+func (q *OpQueue) SetReducer(fn func(ops []*Op, op *Op) []*Op) {
+	q.reducer = fn
+}
+
 // Close releases resources associated with this callgroup, by canceling the context.
 // The owner of this OpQueue should either call Close or cancel the context, both are
 // equivalent.
@@ -112,7 +120,7 @@ func (q *OpQueue) Enqueue(id ID, op *Op) error {
 		return ErrQueueSaturatedWidth
 	}
 
-	set.append(op)
+	set.mergeWith(q.reducer, op)
 	return nil
 }
 
diff --git a/opset.go b/opset.go
index 4a6d957..10a1f8c 100644
--- a/opset.go
+++ b/opset.go
@@ -12,8 +12,11 @@ func newOpSet(op *Op) *OpSet {
 	}
 }
 
-func (os *OpSet) append(op *Op) {
-	os.set = append(os.set, op)
+func appendOp(ops []*Op, op *Op) []*Op {
+	return append(ops, op)
+}
+func (os *OpSet) mergeWith(fn func([]*Op, *Op) []*Op, op *Op) {
+	os.set = fn(os.set, op)
 }
 
 // Ops get the list of ops in this set.
diff --git a/opwindow.go b/opwindow.go
index 6e9ff79..8612a58 100644
--- a/opwindow.go
+++ b/opwindow.go
@@ -27,6 +27,8 @@ type OpWindow struct {
 	depth      int
 	width      int
 	windowedBy time.Duration
+
+	reducer func(ops []*Op, op *Op) []*Op
 }
 
 // NewOpWindow creates a new OpWindow.
@@ -43,11 +45,16 @@ func NewOpWindow(depth, width int, windowedBy time.Duration) *OpWindow {
 		width:         width,
 		windowedBy:    windowedBy,
 		m:             make(map[ID]*queueItem),
+		reducer:       appendOp,
 	}
 	q.q.Init()
 	return q
 }
 
+func (q *OpWindow) SetReducer(fn func(ops []*Op, op *Op) []*Op) {
+	q.reducer = fn
+}
+
 // Close provides graceful shutdown: no new ops will be enqueued.
 func (q *OpWindow) Close() {
 	q.once.Do(func() {
@@ -74,7 +81,7 @@ func (q *OpWindow) Enqueue(ctx context.Context, id ID, op *Op) error {
 				q.mu.Unlock()
 				return ErrQueueSaturatedWidth
 			}
-			item.OpSet.append(op)
+			item.OpSet.mergeWith(q.reducer, op)
 			q.mu.Unlock()
 			return nil
 		}