fact: Definition subspace_ball (x : subspace A) (r : R) := if x \in A then A `&` ball (x : X) r else [set x].
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed