sclass BoolVar extends Var<Bool> {
  *() { super(false); }
  
  public void set(Bool b) {
    super.set(assertNotNull(b));
  }
  
  void waitUntilTrue  { waitForVarToBecome(this, true); }
  void waitUntilFalse { waitForVarToBecome(this, false); }
}