code: public void setMuted(long userId, boolean muted) {
          if (muted) {
              this.muted.add(userId);
          } else {
              this.muted.remove(userId);
          }
      }
docs: Sets the muted state of the user with the given id.
  
  @param userId The id of the user.
  @param muted Whether the user with the given id is muted or not.