generalization `ball_open` #722

Job	Run time
rocq-core	50s
coq	54s
mathcomp-analysis-single	18m 29s
mathcomp-classical	51s
mathcomp-reals	50s
mathcomp-analysis	16m 1s
mathcomp-experimental-reals	42s
mathcomp-reals-stdlib	47s
mathcomp-analysis-stdlib	17m 44s
mathcomp-infotheo	53s
	58m 1s

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