static IVar<S> persistentSecretText(fS persistenceName) {
  ret new IVar<S>() {
    File f = secretProgramFile(urlencode(persistenceName));
    
    public synchronized void set(S a) { saveTextFile(f, a); }
    public synchronized S get() { ret loadTextFile(f); }
    public synchronized bool has() { ret fileExists(f); }
    public synchronized void clear() { set(null); }
  };
}

Began life as a copy of #1009538