Programmable logic controllers (PLC) are used extensively for controlling discrete processes. The PLC's versatility is based on its reprogrammability, but this versatility is bought at the expense of program correctness and verification. Modifications to existing PLC programs, or generation of new ones, requires the use of the controller and the hardware associated with it in order to debug the new instructions. This leads to a greatly increased program design cycle as equipment down-times must be scheduled to accommodate debugging. At the same time, testing uncertain instructions on the plant floor places all the hardware connected to the PLC in jeopardy of damage due to program errors.