fact: Definition mu_ext (X : set T) : \bar R := ereal_inf [set \sum_(k <oo) mu (A k) | A in measurable_cover X].
imports: mathcomp(all_ssreflect all_algebra finmap) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality fsbigop ) reals ereal signed topology normedtype sequences esum numfun HB(structures)