Semantics of shared

[Walter Bright]

Jason House wrote:
> His last blog implied he was further. I thought the recent shift to
> use TLS and shared inside dmd was because a design had been worked
> out. Without that, D hardly helps writing correct multi-threaded code
> :( It may be that I'm being overly pessimistic...

Even if D goes *no further* with shared than where it is now, it still 
helps a *lot* in writing correct multi-threaded code.

It does this by making the points where threads communicate obvious, 
instead of inadvertent and hidden.