code: function setSingle(store, key, value, flag) {
    const { _store, _subscriptions } = store
  
    // Uncurry alfa action functions
    if (typeof value === 'function' && value.alfaAction) {
      value = value.alfaAction(store)
    }
  
    // Save the value to the store.
    _store[key] = value
  
    if (flag === 'merge') {
      return
    }
  
    // Call subscribed functions if we have.
    const subs = _subscriptions[key]
    if (subs) {
      var changed = {
        [key]: value
      }
  
      subs.forEach(function(subFn) {
        // Make sure the subFn is still legit at the time we are calling it since
        // all subscribing functions are actually `setStat`.  And previous
        // `setState` calls could trigger unmount component which later `setState`
        // belongs to.
  
        // Need to make the below code reproducible by test.
        // var _subs = subscriptions[key]
        // if (_subs.indexOf(subFn) === -1) {
        //   return
        // }
  
        if (subFn.maps) {
          var maps = subFn.maps
          Object.keys(maps).some(function(injectKey) {
            const realKey = maps[injectKey]
            if (realKey === key) {
              changed = {
                [injectKey]: value
              }
              return true
            }
            return false
          })
        }
        subFn(changed)
      })
    }
  }
docs: Set a single value to an object.