:f = "f"!script($_)
	|x
	#p!checkpoint($_)
		|a

		#q!checkpoint($_)
			|b

		|c

	|d

|From start:
f!

|From p checkpoint:
f!

|From q checkpoint:
f!

|From q checkpoint again:
f!

|Go to p again by setting checkpoint manually:
f.current checkpoint = #p
f!

|From q again:
f!