fact: Definition open_ball (x0:X) (r:R) : Ensemble X := [ x:X | d x0 x < r ].
imports: Topology(RationalsInReals SubspaceTopology) ZornsLemma(EnsemblesTactics DecidableDec) Coq(Reals ClassicalChoice Program.Subset) Coq(Lia Lra)