code: func (s *State) SetCurrentStep(v *Step) {
  	if v == nil {
  		v = &s.rootStep
  	}
  	if v.s != s {
  		panic("step is not bound to state")
  	}
  	s.stepCursor = v
  }
docs: // SetCurrentStep sets the current step. If the supplied step is nil, the root
  // step will be used.
  //
  // The supplied step must already be registered with the State.