I'll allow myself to ask my second question again: 2) In my algorithm I need to initialize all of the blocks in a graph with bottom element of a lattice, except for the entry block, which needs some other initial values. I've written something like this: cmmCopyPropagation dflags graph = do let entry_blk = g_entry graph g' <- dataflowPassFwd graph [(entry_blk, (Top, Top))] $ analRewFwd cpLattice cpTransfer cpRewrite return . fst $ g' cpLattice = DataflowLattice "copy propagation" (Bottom, Bottom) cpJoin However, it seems that Bottom values passed to cpLattice are ignored - I could replace second parameter to DataflowLattice with `undefined` and the code would still run without causing an error. Is there something obviously wrong in the way I pass initial fact values to dataflowPassFwd, or should I look for the problem in other parts of my code? Janek