text: export const ADD_TODO = 'ADD_TODO';
  export const TOGGLE_TODO = 'TOGGLE_TODO';
  export const REMOVE_TODO = 'REMOVE_TODO';
  export const SHOW_ALL = 'SHOW_ALL';
  export const SHOW_COMPLETED = 'SHOW_COMPLETED';
  export const SHOW_ACTIVE = 'SHOW_ACTIVE';
  export const SET_VISIBILITY_FILTER = 'SET_VISIBILITY_FILTER';