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.