copy only reference rather duplicate a string in appender!string

[Marc]

I do build a string by coping large parts of diffrent buffers, 
all those buffers live after the functional call, so rather than 
duplicate those string I'd like to copy only references to those 
parts rather duplicate every string. I combined appender!string, 
assumeUnique() and array slices. Something like this:

> auto buffer = appender!string;
> auto list = cycle(buffers);
> while(bufferIsFilled) {
>   char[] buffer = get_buffer(); // pop a buffer from circular 
> buffer
>   int s = get_buffer_size(x); // determine which part of that 
> buffer we need
>   buffer.put(assumeUnique(buf[0 .. s])); // and here's my 
> question
> }
> return buffer.data;