$ f
	x
	§ p
		a

		§ q
			b

		c

	d

From start:
~ f

From p checkpoint:
~ f

From q checkpoint:
~ f

From q checkpoint again:
~ f

Force p checkpoint:
~ f.p()

From q again:
~ f

Go to p again by setting checkpoint manually:
~ f.checkpoint := &f.p
~ f