const state1 = countReducer(undefined, {TYPE: '@@redux/INIT'})
// the initial state was set in defaultState and its value is 100
// ⇒ state1 === 100

const state2 = countReducer(state1, increaseBy(100))
// ⇒ state2 === 200

const state3 = countReducer(state2, increaseBy(-500))
// ⇒ state3 === -300