workflow 4.0的教材，英文版本。
原文介绍：
The Workflow Foundation included in .Net 4.0 (referred to as WF 4.0) represents a whole
new paradigm for building workflow-based applications. It has been completely reengineered
from the ground up. In this section, you’ll design some simple workflows and
learn the basic concepts. In subsequent sections, you’ll develop more complex solutions as
you explore the capabilities provided by WF 4.0.
