type StateMachine interface {
  void Enter();
  void Update();
  void Exit();
}

type DefaultStateMachine struct {}

void (dsm *DefaultStateMachine) Enter() {}
void (dsm *DefaultStateMachine) Update() {}
void (dsm *DefaultStateMachine) Exit() {}

... (meanwhile in another package) ...

type UserStateMachine struct {
    gofsm.DefaultStateMachine
}

void (usm *UserStateMachine) Update() {
  // User code here, adheres to the interface without forcing methods, etc.
}