Set Operations
There is a natural interpretation of the set operations (union, intersection, difference, etc.) in combining data flow information
If two control paths meet to enter the top of a given basic block, then the set of definitions that reach that block is the union of the true definitions in the boolean vectors on the incoming paths
Likewise, the definitions that obtain after execution of a basic block can be computed by taking the intersection of the incoming definitions and the PRESERVED set for the block and unioning this with the EXPOSED set for the block