Is there any good reason why C++ namespaces are "closed" in D?

[rikki cattermole]

On 03/08/2018 9:12 AM, Walter Bright wrote:
> On 8/2/2018 2:05 AM, rikki cattermole wrote:
>> 8. if any identifier starts with a keyword and ends with at minimum 
>> one _, one _ from the end of the identifier will be removed for 
>> mangling (but not e.g. lookup).
> 
> This will break existing code. A double underscore prefix is reserved 
> for the implementation, which is why I went down that path.

Because it will affect mangling only, do we have any examples of c/c++ 
code that appends _'s to it that is used by the D community?