|16||16|| * in forward analysis, if user passes in facts about all entry points to the graph, then we don't need `fact_bot` field of `DataflowLattice`, because we will always have some fact about analysed block. What a user needs to pass is `[(L,F)]`, where `L` is type of label of an entry point and `F` is a type of fact about that entry point. If user provides a list of facts about every entry label then we don't need a separate list of entry points to the graph, because we can recover that from a list of facts.
|17||17|| * in backward analysis we still need user to supply a list `[L]` of entry points to the graph. We use them to determine a group of live blocks and then analyze the graph starting from its exit points. For a graph closed on exit we cannot know anything about these exit points, so we don't supply any known facts. We have to supply a bottom element `F`, which will be default initial value for every exit point in backward analysis. Graphs open on exit are more trickier, because if the graph is open on exit then we should know something about that exit point. Moreover, we still need a bottom element, because graph may have more than one exit point.