Add requirement for ContainerBox.Add to remove a previously

parented object first

object.go

@@ -195,11 +195,11 @@ type CanvasBox interface {
 	Box
 
 	// SetDrawer sets the Drawer that will be called upon to draw the Box's
-	// content when it is invalidated. The canvas passed to the drawer will
+	// content when it is invalidated. The Canvas passed to the drawer will
 	// have these properties:
 	//   - It will have the same origin (0, 0) as the window which contains
 	//     the CanvasBox
-	//   - The canvas bounds will describe the portion of the CanvasBox
+	//   - The Canvas bounds will describe the portion of the CanvasBox
 	//     visible on screen
 	SetDrawer (canvas.Drawer)
 
@@ -265,7 +265,8 @@ type ContainerBox interface {
 
 	// SetGap sets the gap between child Objects.
 	SetGap             (image.Point)
-	// Add appends a child Object.
+	// Add appends a child Object. If the object is already a child of
+	// another object, it will be removed from that object first.
 	Add                (Object)
 	// Delete removes a child Object, if it is a child of this Box.
 	Delete             (Object)