void ChangeStatus() {
	SaveFile(GetExeDirFile("lock"), "Finished succesfully");
}

if(!FileExists(GetExeDirFile("lock")))
	   SaveFile(GetExeDirFile("lock"), "Finished sucessfully");
	   
	String status = LoadFile(GetExeDirFile("lock"));
	
	if(status == "running")
	{
		Exclamation("There's another instance!");
		return;
	}
	
	if(status == "crashed") {
		Exclamation("The program crashed on the last run. Please restart the application.");
		RecoverData();
		ChangeStatus();
		return;
	}
	
	SaveFile(GetExeDirFile("lock"), "running");
	
	atexit(ChangeStatus);

SaveFile(GetExeDirFile("lock"), "crashed");
DoLongOperations(); // if it crashes now we're safe
SaveFile(GetExeDirFile("lock"), "running"); // We survived