public void removeUser(String username) {
    // TODO: handle if user doesn't exist!

    User removeUser = this.userStorage.remove(username);

    for (Mumble mumble : removeUser.getMumbles()) {
      this.mumbleStorage.remove(mumble.getId());
      this.mumbleTimeline.remove(mumble);
    }
  }
  public void removeMumble(String mumbleId) {
    // TODO: handle if mumble doesn't exist!

    Mumble removedMumble = this.mumbleStorage.remove(mumbleId);
    this.mumbleTimeline.remove(removedMumble);

    // remove the mumble from the User!
    User user = this.userStorage.get(removedMumble.getUsername());
    user.getMumbles().remove(removedMumble);
  }
  public Mumble newMumble(String username, String message) {
    User user = this.userStorage.get(username);

    // TODO: what if user doesn't exist?

    Mumble newMumble = new Mumble(username, message);
    user.addMumble(newMumble);
    this.mumbleStorage.put(newMumble.getId(), newMumble);
    this.mumbleTimeline.add(newMumble);

    return newMumble;
  }
  public void updateUser(User user) {
    // TODO: handle if user doesn't exist!

    this.userStorage.put(user.getUsername(), user);
  }
  public void addUser(User user) {
    // TODO: what if we already have that username?

    this.userStorage.put(user.getUsername(), user);
  }