To interface or not to interface

[Steven Schveighoffer]

On Mon, 24 May 2010 18:13:38 -0400, Walter Bright  
<newshound1 at digitalmars.com> wrote:

> Steven Schveighoffer wrote:
>> All an interface does is give an abstract representation of functions  
>> that are *already there*.  Removing the interface does not remove the  
>> functions that implemented the interface.
>
> Then why do interfaces need to be part of the collection component? Why  
> can't the user add them if he wants them?

How do you add an interface to a class?  Wrapping seems like it would add  
more overhead than just implementing the interface, especially since D's  
inliner has some strange restrictions.

-Steve