Design a synchronous internal controller for a $4-$floor elevator. The elevator remains on the ground floor in the idle state. There are three input keys inside the elevator to indicate first, second and third floor. In addition, the respective floor key is also present on its floor to call the elevator on that floor. The movement of the elevator from a floor to other is possible by pressing $($asserting$)$ the key for the desired floor. However, transition to the ground floor from any floor is only possible when none of the three keys are pressed $($all keys are non$-$asserted$).$ Elevator will remain on the same floor if the key corresponding to that floor is pressed again after reaching that floor $($for ex$.,$ the elevator will remain on the third floor after reaching there, if key $3$ is pressed at third floor again$)$ or multiple keys are pressed together.