ScopedValue API

[Peter Firmstone]

Just wanted to say, I've been experimenting with replacing ThreadLocal 
with ScopedValue, this is a great new API, I love the way ScopedValue 
uses recursion, we can have multiple immutable instances representing a 
scoped value for a short period and we're not worried about managing 
state as it goes out of scope as soon as it's no longer in use.

Cheers,
Peter.