sentences:  The machine remains stuck in the power-up state or can be moved only between other can't-happen states in a walled garden of states.This is also called a "hardware lockup" or "soft error".Such states, while nominally can't-happen, are not don't-care, and designers take steps either to ensure that they are really made can't-happen, or else if they do happen, that they create a "don't-care alarm" indicating an emergency state for error detection, or they are transitory and lead to a normal operational state.