A non-transitory computer readable medium storing instructions for identifying at least one improper or unknown operation of a device under test, DUT, based on an emulation by an emulator of the DUT comprising a flip flop, the instructions when executed configure the emulator to: convert the DUT from a two state semantic having one of a high state and a low state using one bit representing a bit signal into a four state semantic having one of the high state, the low state, an unknown state and an unassigned state using two bits (410C, 410D), wherein a first of the two bits and a second of the two bits each having the high state represents the unknown state of the bit signal in the four state semantic, the first bit having the high state and the second bit having the low state represents the high state of the bit signal in the four state semantic, the first bit having the low state and the second bit having the high state represents the low state of the bit signal in the four state semantic, and the first bit and the second bit each having the low state represents the unassigned state of the bit signal in the four state semantic; during the emulation of the DUT having the four state semantic, receive, at the flip flop, an input signal having the four state semantic and a reference signal comprising reference bits (620A, 620B) and having the four state semantic; determine, based on a transistion state bit and the reference bits (620A, 620B), whether the reference signal has a transition in a state of the reference signal from an unknown state or to the unknown state; generate, responsive to determining the reference signal having the transition, an output of the flip flop having the four state semantic based on a state of the input signal and a state of the output of the of the flip flop prior to the transition of the reference signal, the state of the output of the flip flop being maintained when the state of the input signal is the same as the state of the output of the flip flop prior to the transition of the reference signal, and the state of the output of the flip flop becoming the unknown state when the state of the input signal and the state of the output of the flip flop prior to the transition of the reference signal are different; propagate the unknown state to an output of the DUT having the four state semantic by performing a digital logic operation using the output of the flip flop as an input of a digital logic circuit of the DUT having the four state semantic; and identify at least one improper or unknown operation of the DUT having the four state semantic in response to the output of the DUT having the unknown state. convert the DUT from a two state semantic having one of a high state and a low state using one bit representing a bit signal into a four state semantic having one of the high state, the low state, an unknown state and an unassigned state using two bits (410C, 410D), wherein a first of the two bits and a second of the two bits each having the high state represents the unknown state of the bit signal in the four state semantic, the first bit having the high state and the second bit having the low state represents the high state of the bit signal in the four state semantic, the first bit having the low state and the second bit having the high state represents the low state of the bit signal in the four state semantic, and the first bit and the second bit each having the low state represents the unassigned state of the bit signal in the four state semantic; during the emulation of the DUT having the four state semantic, receive, at the flip flop, an input signal having the four state semantic and a reference signal comprising reference bits (620A, 620B) and having the four state semantic; determine, based on a transistion state bit and the reference bits (620A, 620B), whether the reference signal has a transition in a state of the reference signal from an unknown state or to the unknown state; generate, responsive to determining the reference signal having the transition, an output of the flip flop having the four state semantic based on a state of the input signal and a state of the output of the of the flip flop prior to the transition of the reference signal, the state of the output of the flip flop being maintained when the state of the input signal is the same as the state of the output of the flip flop prior to the transition of the reference signal, and the state of the output of the flip flop becoming the unknown state when the state of the input signal and the state of the output of the flip flop prior to the transition of the reference signal are different; propagate the unknown state to an output of the DUT having the four state semantic by performing a digital logic operation using the output of the flip flop as an input of a digital logic circuit of the DUT having the four state semantic; and identify at least one improper or unknown operation of the DUT having the four state semantic in response to the output of the DUT having the unknown state.