`is_deriveX`: is that a generalization? #617

Job	Run time
coq	48s
mathcomp-classical	2m 52s
mathcomp-analysis-single	12m 48s
mathcomp-reals	1m 52s
mathcomp-experimental-reals	1m 45s
mathcomp-analysis	10m 55s
mathcomp-reals-stdlib	1m 29s
mathcomp-analysis-stdlib	2m 24s
ssprove	5m 31s
	40m 24s

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