generalization `ball_open` #64

Job	Run time
rocq-core	44s
mathcomp	46s
coq	46s
mathcomp-analysis-single	14m 37s
mathcomp-classical	48s
mathcomp-reals	2m 1s
mathcomp-experimental-reals	1m 24s
mathcomp-reals-stdlib	43s
mathcomp-analysis	12m 33s
ssprove	45s
mathcomp-analysis-stdlib	50s
mathcomp-infotheo	6m 57s
	42m 54s

Provide feedback