The Formal Semantics of Computer Languages and their Implementations

ions which did not contain assignments to their formalparameters, but the necessary changes to the predicates are ex­tensive and do not yield a wider class of computations: if I isnot covertly assigned to in L 2.5.3 establishes that E is equivalentto e[E](i\I'.I'=I~false,tpue), and thus a version of 2.5.4 confirms that fnI.E can be replaced by fnI .• E, which we allow in any case.Similar considerations govern declarations by reference which arenot recursive, for unless I=E is equivalent to I=$E it may latergive rise to an illegitimate assignment. As recursive declarationsare still permitted x=$y, say, can be replaced by rec x=y.Abstractions by incidence must be severely constrainedbecause they are used both 1n function applications and as ex­pression exits. A failure to confine their free variables to thosewhich can be passed out of the surrounding context can lead to adisaster like 3.1.5.It is convenient to prohibit get E, val E and A inside Efrom appearing as exits on the left hand sides of assignment state­ments. We could devise a function which would list all the 'secondary' exits from the exits of an expression; were this to208