Boreale, Michele and De Nicola, Rocco and Pugliese, Rosario  A Theory of "May" Testing for Asynchronous Languages.   In:  Foundations of Software Science and Computation Structures (FoSSaCS 1999).   Lecture Notes in Computer Science, 1578 .  Springer, pp. 165-179.  ISBN 3-540-65719-3      (1999)