Add requirement for ContainerBox.Add to remove a previously

parented object first

object.go

@@ -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)