if (PREF_UnregisterCallback(domain, callback, instance_data) == PREF_NOERROR) { return NS_OK; } else { return NS_ERROR_FAILURE; }
return PREF_UnregisterCallback(domain, callback, instance_data);
NS_IMETHODIMP nsPref::UnregisterCallback( const char* domain,                        PrefChangedFunc callback,                         void* instance_data )//----------------------------------------------------------------------------------------{  if (PREF_UnregisterCallback(domain, callback, instance_data) == PREF_NOERROR) {    return NS_OK;  } else {    return NS_ERROR_FAILURE;  }}