This is a state machine for the Engine. The state machine manages a set of states,
each of which has a set of System providers. When the state machine changes the state, it removes
Systems associated with the previous state and adds Systems associated with the new state.
This is a state machine for the Engine. The state machine manages a set of states, each of which has a set of System providers. When the state machine changes the state, it removes Systems associated with the previous state and adds Systems associated with the new state.