function assertCurrentStep<S extends State['step']>(
  state: State,
  step: S,
): asserts state is Extract<State, {step: S}> {
  if (state.step !== step) {
    throw new InvalidStateTransitionError(
      `Invalid state transition: expecting ${step}, got ${state.step}`,
    )
  }
}

class InvalidStateTransitionError extends Error {
  constructor(message: string) {
    super(message)
    this.name = 'InvalidStateTransitionError'
  }
}

// For use in a reducer - means that you can ensure that
// a transition between two states A and B can only take place
// starting in state A, and provides typesafety
// function reducer(state: State, action: Action): State {
//   switch (action.type) {
//     case 'GO_TO_STEP_2': {
//       assertCurrentStep(state, 'step 1')
//       return {
//         step: 'step 2',
//         ...action.payload,
//       }
//     }
//   }
// }