vm/state.go
func (st *State) PopFrame() *frame.Frame {
	x := st.frames.Pop()
	if x == nil {
		return nil
	}
	f := x.(*frame.Frame)
	for i := st.framestack.Size(); i > f.Mark(); i-- {
		st.framestack.Pop()
	}
	return f
}
func (st *State) PopFrame() *frame.Frame {
	x := st.frames.Pop()
	if x == nil {
		return nil
	}
	f := x.(*frame.Frame)
	for i := st.framestack.Size(); i > f.Mark(); i-- {
		st.framestack.Pop()
	}
	return f
}
// PopFrame pops the frame from the top of the frame stack