1. Apparatus for simulating the behavior of a digital circuit, the digital circuit having a plurality of connected circuit elements, said apparatus having means for defining said circuit elements comprising means for defining inputs to said circuit elements having at least four signal levels, said levels including a "0" signal level, a "1" signal level, a high impedance "Z" signal level, and an unknown "X" signal level, means connected to said input defining means for defining outputs of said circuit elements based upon said input signals, and said outputs having at least said four signal levels, and said output defining means including means for providing a "0" or a "1" output signal level even though one or more input signal levels have thereon a "Z" or an "X" signal level.