function isMobile(ctx) {
  return ctx.screen === 'mobile'
}

const updateOrientation = assign({
  screen: ctx => ctx.screen === 'desktop' ? 'mobile' : 'desktop'
});

const machine = Machine({
  initial: 'X',
  context: {
    screen: 'desktop'
  },
  states: {
    X: {
      on: {
        NEXT: [
          { target: 'AB', cond: isMobile },
          { target: 'A' }
        ],
        REORIENT: { actions: updateOrientation }
      }
    },
    A: {
      on: {
        BACK: 'X',
        NEXT: 'B',
        REORIENT: { actions: updateOrientation }
      }
    },
    B: {
      on: {
        BACK: 'A',
        NEXT: 'Y',
        REORIENT: {
          target: 'AB',
          actions: updateOrientation
        }
      }
    },
    AB: {
      on: {
        BACK: 'X',
        NEXT: 'Y',
        REORIENT: { actions: updateOrientation }
      }
    },
    Y: {
      type: 'final',
    }
  }
})