  Separation Logic is a non-classical logic used to verify pointer-intensive
code. In this paper, however, we show that Separation Logic, along with its
natural extensions, can also be used as a specification language for
concurrent-system design. To do so, we express the behavior of three very
different concurrent systems: a Subway, a Stopwatch, and a 2x2 Switch. The
Subway is originally implemented in LUSTRE, the Stopwatch in Esterel, and the
2x2 Switch in Bluespec.
