text: import React from 'react';
  import { createStackNavigator } from 'react-navigation-stack';
  
  import MainScreen from '../screens/MainScreen';
  import SettingScreen from '../screens/SettingScreen';
  import AboutScreen from '../screens/AboutScreen';
  import {AppStackNavigatorMath} from './AppStackNavigatorMath';
  
  export const AppStackNavigator = createStackNavigator({
     MathScreen : {
      screen : AppStackNavigatorMath,
      navigationOptions:{
        headerShown : false
      }
    },
    SettingScreen : {
      screen : SettingScreen,
      navigationOptions:{
        headerShown : false
      }
    },
    AboutScreen :{
      screen : AboutScreen,
      navigationOptions:{
        headerShown : false
      }
    }
  },
    {
      initialRouteName: 'MainScreen'
    }
  );
  