but when we state that A ''equals'' B , as well having to know what we mean by A and B we also have know what we mean by ''equals''. This section explores the role of observers; how different types of observ­ er see different things as being equal, and how we can produce algo­ rithms to decide on such equalities. It also explores how we go about writing specifications to which we may compare our SCCS designs. • The final section is the one which the students like best. Once enough of SCCS is grasped to decide upon the component parts of a design, the ''turning the handle'' steps of composition and check­ ing that the design meets its specification are both error-prone and tedious. This section introduces the concurrency work bench, which shoulders most of the burden. How you use the book is up to you; I''m not even going to suggest path­ ways. Individual readers know what knowledge they seek, and course leaders know which concepts they are trying to impart and in what order.