func (kv *kvStore) Append(id string, value []byte) error {
	kv.lock.Lock()
	defer kv.lock.Unlock()
	kv.store[id] = append(kv.store[id], value...)
	return nil
}
Append a value to any existing value at key id.
package main
import "sync"
type kvStore struct {
	store map[string][]byte
	lock  sync.RWMutex
}
func newKVStore() (*kvStore, error) {
	return &kvStore{
		store: make(map[string][]byte),
	}, nil
}
func (kv *kvStore) Put(id string, value []byte) error {
	kv.lock.Lock()
	defer kv.lock.Unlock()
	kv.store[id] = value
	return nil
}