We augment B’s existing total-correctness semantics of weakest precondition (wp) with a partial-correctness semantics of weakest liberal precondition (wlp). By so doing we achieve a general-correctness semantics for B operations which not only accords more fully with our natural computational intuition, but also extends the essential expressive capability of B’s Generalised Substitution Language (GSL) to embrace a whole new class of operations called semi-decidable, whose behaviour cannot be characterised in terms of total correctness alone. The ability to specify semi-decidable operations is important because a desired conventional operation may lend itself to implementation as a concurrent federation of semi-decidable operations co-operating under a mutual “termination pact”. Indeed, computational constraints may render this the only viable implementation strategy. We call a generalised substi- tution invested with our general-correctness semantics an abstract command. Our Abstract Command Language (ACL) is thus syntactically indistinguishable from the GSL, save for the introduction of one new composition operator, concert, expressing a “termination pact” between two concurrent abstract commands.