allow stack_change to be negative #409

Job	Run time
ci / Build	3m 40s
ci / Typecheck	54s
ci / Lint	58s
ci / Test (0)	1m 26s
ci / Test (1)	2m 1s
ci / Test (2)	1m 45s
ci / Test (3)	1m 9s
ci / Test (4)	1m 54s
ci / Test (5)	1m 0s
ci / Test (6)	1m 36s
ci / Test (7)	1m 28s
ci / Test (8)	3m 29s
ci / Test (9)	1m 11s
ci / Publish Unit Tests Results	1s
	22m 32s

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