Re: X3 thread-safety

> > But in the new version obj is taken by reference and forwarded to
> > with_value_holder!
>
> Ooh that's subtle. And good to know. For me it's nice, because I usually
> had to bother with `reference_wrapper` or similar kludges

It's nice we can pass an object by reference. It's much less nice that
one should now bend over backwards to employ value semantics (which
seems to me rather common scenario).
Maybe it's worth making separate with's for these two tasks, like
"with" and "with_ref"?