|:: m f ()||Useful for starting a new iteration
|:: m f a|
|-> m f a||Do a new analysis and then throw away
all the related state.
|It might be surprising these next two are needed in a pure analysis,
but for some problems we do a shallow rewriting in which a rewritten
graph is not itself considered for further rewriting but merely undergoes
an analysis. In this case the results of a forward analysis might produce
new facts that go on BlockId's that reside outside the graph being analyzed.
Thus these lastOutFacts need to be available even in a pure analysis.