When you assigning the worker in TreeRange, you create a delegate
that captures the current TreeRange or 'this'.
---
worker = new Fiber(&fiberFunc);
---
foreach is defined as
(http://dlang.org/statement.html#ForeachStatement):
---
for (auto __r = range; !__r.empty; __r.popFront())
{
auto e = __r.front;
...
}
---
__r is a copy of your range that still refers to the original
worker which in turn refers to your original range. So when you
popFront, the worker will advance the original range and if you
call front you get the element from __r - unadvanced.
That's my best guess.
