code: func (ks *KeyStore) Lock(addr common.Address) error {
  	ks.mu.Lock()
  	if unl, found := ks.unlocked[addr]; found {
  		ks.mu.Unlock()
  		ks.expire(addr, unl, time.Duration(0)*time.Nanosecond)
  	} else {
  		ks.mu.Unlock()
  	}
  	return nil
  }
docs: // Lock removes the private key with the given address from memory.