The importance of giving precise semantics to programming and specification<br />languages was recognized since the sixties with the development of the<br />first high-level programming languages (cf. e.g. [30, 206] for some early accounts).<br />The use of operational semantics - i.e. of a semantics that explicitly<br />describes how programs compute in stepwise fashion, and the possible<br />state-transformations they perform - was already advocated by McCarthy<br />in [147], and elaborated upon in references like [142, 143]. Examples of full-blown<br />languages that have been endowed with an operational semantics are<br />Algol 60 [140], PL/I [173], and CSP [178].