[libsigc++2/variadic_bind4: 14/18] Add a TODO comment.

[Murray Cumming <murrayc src gnome org>]

commit 3bd0819e4e6ada9c8edef297d94753d8398aebf4
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Feb 19 13:06:22 2016 +0100

    Add a TODO comment.

 sigc++/adaptors/bind.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/adaptors/bind.h b/sigc++/adaptors/bind.h
index 3f5cf32..5816c19 100644
--- a/sigc++/adaptors/bind.h
+++ b/sigc++/adaptors/bind.h
@@ -94,6 +94,7 @@ template <class T_element>
 struct TransformEachInvoker
 {
   //We take T_element as non-const because invoke() is not const.
+  //TODO: Take element as T_element&& ?
   static
   decltype(auto)
   transform(T_element& element) {