Concurrency.

[Dejan Lekic]

> 
>>>Not having default sharing is about providing guaranties, it's about
>>>making thread-safety checkable by the compiler.
> 
> No piece of code can absolutely guarantee threading errors. When you start

Threading errors and thread-safety checks are different things... It is 
pretty much possible to do thread-safety checks perhaps not all, but 
compiler tries what it can do to "shout" back at developer when an error is 
made. :)