fact: Definition nbhs_subspace (x : subspace A) : set_system (subspace A) := if x \in A then within A (nbhs x) else globally [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