diff --git a/engine/game.go b/engine/game.go
index 71cada6..548d048 100644
--- a/engine/game.go
+++ b/engine/game.go
@@ -3,6 +3,7 @@ package engine
 import (
 	"encoding/gob"
 	"io/fs"
+	"log"
 	"sync"
 
 	"github.com/hajimehoshi/ebiten/v2"
@@ -59,6 +60,9 @@ func (g *Game) RegisterComponent(c interface{}) {
 		return
 	}
 	g.dbmu.Lock()
+	if _, exists := g.db[id]; exists {
+		log.Printf("duplicate id %q", id)
+	}
 	g.db[id] = c
 	g.dbmu.Unlock()
 }
diff --git a/game/aw.go b/game/aw.go
index dba5d33..7be4b0f 100644
--- a/game/aw.go
+++ b/game/aw.go
@@ -10,11 +10,24 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/inpututil"
 )
 
+var (
+	_ engine.Identifier  = &Awakeman{}
+	_ engine.Drawer      = &Awakeman{} // provided by Sprite
+	_ engine.DrawOrderer = &Awakeman{} // provided by Sprite
+	_ engine.Disabler    = &Awakeman{}
+	_ engine.Hider       = &Awakeman{} // provided by Sprite
+	_ engine.Prepper     = &Awakeman{}
+	_ engine.Scanner     = &Awakeman{}
+	_ engine.Updater     = &Awakeman{}
+)
+
 func init() {
 	gob.Register(&Awakeman{})
 }
 
 type Awakeman struct {
+	engine.ID
+	engine.Disabled
 	engine.Sprite
 
 	CameraID string
@@ -32,6 +45,10 @@ type Awakeman struct {
 }
 
 func (aw *Awakeman) Update() error {
+	if aw.Disabled {
+		return nil
+	}
+
 	// TODO: better cheat for noclip
 	if inpututil.IsKeyJustPressed(ebiten.KeyN) {
 		aw.noclip = !aw.noclip
diff --git a/main.go b/main.go
index 647112b..4c99869 100644
--- a/main.go
+++ b/main.go
@@ -94,6 +94,45 @@ func repl(g *engine.Game) {
 			g.Prepare()
 			g.Enable()
 			g.Show()
+		case "tree":
+			var c interface{} = g
+			if len(tok) == 2 {
+				// subtree
+				id := tok[1]
+				c = g.Component(id)
+				if c == nil {
+					log.Printf("Component %q not found", id)
+					break
+				}
+			}
+			type item struct {
+				c     interface{}
+				depth int
+			}
+			stack := []item{{c, 0}}
+			for len(stack) > 0 {
+				tail := len(stack) - 1
+				x := stack[tail]
+				stack = stack[:tail]
+				c := x.c
+
+				indent := ""
+				if x.depth > 0 {
+					indent = strings.Repeat("  ", x.depth-1) + "↳ "
+				}
+				i, ok := c.(engine.Identifier)
+				if ok {
+					log.Printf("%s%T %s", indent, c, i.Ident())
+				} else {
+					log.Printf("%s%T", indent, c)
+				}
+
+				if s, ok := c.(engine.Scanner); ok {
+					for _, y := range s.Scan() {
+						stack = append(stack, item{y, x.depth + 1})
+					}
+				}
+			}
 		}
 	}
 	if err := sc.Err(); err != nil {