BasicBlock is a record to hold the data of a basic block of the CDFG of a cmd program. rdVars and wrVars hold the sets of variables that were read and written by that basic block, cmdProg is the program of that basic block, and actDepGraph is its action dependency graph. nextBBs is the set of successor basic blocks (either one or two). Finally, lineBegin and lineEnd are the first and last lines of the cmd program that belong to this basic block.