diff --git a/lib/lru/README.md b/lib/lru/README.md
index ec400c1..2c06be4 100644
--- a/lib/lru/README.md
+++ b/lib/lru/README.md
@@ -1,3 +1,3 @@
 # Lru
 
-Package lru implements an LRU cache.
+Package lru implements an lru cache.
diff --git a/lib/lru/lru.go b/lib/lru/lru.go
index 9612138..fe097f0 100644
--- a/lib/lru/lru.go
+++ b/lib/lru/lru.go
@@ -1,3 +1,4 @@
+// A data structure that follows the constraints of a least recently used (lru) cache.
 package lru
 
 import (
diff --git a/lib/priority/README.md b/lib/priority/README.md
new file mode 100644
index 0000000..2895c1e
--- /dev/null
+++ b/lib/priority/README.md
@@ -0,0 +1,3 @@
+# Priority
+
+Package priority implements a priority mutex.
diff --git a/lib/priority/priority.go b/lib/priority/priority.go
new file mode 100644
index 0000000..de49704
--- /dev/null
+++ b/lib/priority/priority.go
@@ -0,0 +1,29 @@
+package priority
+
+import (
+	"sync"
+)
+
+// Priority implement a lock with priorities.
+type Priority struct {
+	l []sync.Mutex
+}
+
+// Call the function f with priority.
+func (p *Priority) Pri(n int, f func() error) error {
+	for i := n; i >= 0; i-- {
+		p.l[i].Lock()
+	}
+	err := f()
+	for i := n; i >= 0; i-- {
+		p.l[i].Unlock()
+	}
+	return err
+}
+
+// NewPriority returns a new Priority with n priority levels.
+func NewPriority(n int) *Priority {
+	return &Priority{
+		l: make([]sync.Mutex, n),
+	}
+}
diff --git a/lib/priority/priority_test.go b/lib/priority/priority_test.go
new file mode 100644
index 0000000..64c48f0
--- /dev/null
+++ b/lib/priority/priority_test.go
@@ -0,0 +1,27 @@
+package priority
+
+import (
+	"testing"
+)
+
+func BenchmarkPriority(b *testing.B) {
+	pri := NewPriority(3)
+	for range b.N {
+		pri.Pri(2, func() error {
+			return nil
+		})
+	}
+}
+
+func TestPriority(t *testing.T) {
+	pri := NewPriority(3)
+	pri.Pri(0, func() error {
+		return nil
+	})
+	pri.Pri(1, func() error {
+		return nil
+	})
+	pri.Pri(2, func() error {
+		return nil
+	})
+}
diff --git a/protocol/czar/mux.go b/protocol/czar/mux.go
index f61b987..45419c8 100644
--- a/protocol/czar/mux.go
+++ b/protocol/czar/mux.go
@@ -7,6 +7,7 @@ import (
 	"sync"
 
 	"github.com/mohanson/daze/lib/doa"
+	"github.com/mohanson/daze/lib/priority"
 )
 
 // A Stream managed by the multiplexer.
@@ -26,7 +27,7 @@ func (s *Stream) Close() error {
 	s.rer.Put(io.ErrClosedPipe)
 	s.wer.Put(io.ErrClosedPipe)
 	s.zo0.Do(func() {
-		s.mux.pri.H(func() error {
+		s.mux.pri.Pri(0, func() error {
 			s.mux.con.Write([]byte{s.idx, 0x02, 0x00, 0x00})
 			return nil
 		})
@@ -39,7 +40,7 @@ func (s *Stream) Esolc() error {
 	s.rer.Put(io.EOF)
 	s.wer.Put(io.ErrClosedPipe)
 	s.zo0.Do(func() {
-		s.mux.pri.H(func() error {
+		s.mux.pri.Pri(0, func() error {
 			s.mux.con.Write([]byte{s.idx, 0x02, 0x01, 0x00})
 			return nil
 		})
@@ -98,7 +99,7 @@ func (s *Stream) Write(p []byte) (int, error) {
 		binary.BigEndian.PutUint16(b[2:4], uint16(l))
 		copy(b[4:], p[:l])
 		p = p[l:]
-		err := s.mux.pri.M(func() error {
+		err := s.mux.pri.Pri(1, func() error {
 			if err := s.wer.Get(); err != nil {
 				return err
 			}
@@ -144,7 +145,7 @@ type Mux struct {
 	ach chan *Stream
 	con net.Conn
 	idp *Sip
-	pri *Priority
+	pri *priority.Priority
 	rer *Err
 	usb []*Stream
 }
@@ -171,7 +172,7 @@ func (m *Mux) Open() (*Stream, error) {
 	if err != nil {
 		return nil, err
 	}
-	err = m.pri.H(func() error {
+	err = m.pri.Pri(0, func() error {
 		return doa.Err(m.con.Write([]byte{idx, 0x00, 0x00, 0x00}))
 	})
 	if err != nil {
@@ -250,7 +251,7 @@ func NewMux(conn net.Conn) *Mux {
 		ach: make(chan *Stream),
 		con: conn,
 		idp: NewSip(),
-		pri: NewPriority(),
+		pri: priority.NewPriority(2),
 		rer: NewErr(),
 		usb: make([]*Stream, 256),
 	}
diff --git a/protocol/czar/priority.go b/protocol/czar/priority.go
deleted file mode 100644
index 8e0df12..0000000
--- a/protocol/czar/priority.go
+++ /dev/null
@@ -1,48 +0,0 @@
-package czar
-
-import (
-	"sync"
-)
-
-// Priority implement a lock with three priorities.
-type Priority struct {
-	l *sync.Mutex
-	m *sync.Mutex
-	h *sync.Mutex
-}
-
-// H executes function f with 0 priority.
-func (p *Priority) H(f func() error) error {
-	p.h.Lock()
-	defer p.h.Unlock()
-	return f()
-}
-
-// H executes function f with 1 priority.
-func (p *Priority) M(f func() error) error {
-	p.m.Lock()
-	defer p.m.Unlock()
-	p.h.Lock()
-	defer p.h.Unlock()
-	return f()
-}
-
-// H executes function f with 2 priority.
-func (p *Priority) L(f func() error) error {
-	p.l.Lock()
-	defer p.l.Unlock()
-	p.m.Lock()
-	defer p.m.Unlock()
-	p.h.Lock()
-	defer p.h.Unlock()
-	return f()
-}
-
-// NewPriority returns a new Priority.
-func NewPriority() *Priority {
-	return &Priority{
-		l: &sync.Mutex{},
-		m: &sync.Mutex{},
-		h: &sync.Mutex{},
-	}
-}
diff --git a/protocol/czar/priority_test.go b/protocol/czar/priority_test.go
deleted file mode 100644
index 1258837..0000000
--- a/protocol/czar/priority_test.go
+++ /dev/null
@@ -1,18 +0,0 @@
-package czar
-
-import (
-	"testing"
-)
-
-func TestProtocolCzarPriority(t *testing.T) {
-	pri := NewPriority()
-	pri.H(func() error {
-		return nil
-	})
-	pri.M(func() error {
-		return nil
-	})
-	pri.L(func() error {
-		return nil
-	})
-}