Here we define $\wpre{\expr'}{\Ret\var.\prop}[\mask]\eqdef\TRUE$ if $\expr' =\bot$ (remember that our stepping relation can, but does not have to, define a forked-off expression).
Here we define $\wpre{\expr'}[\mask]{\Ret\var.\prop}\eqdef\TRUE$ if $\expr' =\bot$ (remember that our stepping relation can, but does not have to, define a forked-off expression).
% The following lemmas help in proving axioms for a particular language.
% The first applies to expressions with side-effects, and the second to side-effect-free expressions.
% \dave{Update the others, and the example, wrt the new treatment of $\predB$.}