let X be non empty non almost_discrete TopSpace; :: thesis: for A0 being non empty Subset of X st A0 is boundary & A0 is closed holds
ex X0 being non empty strict closed boundary SubSpace of X st A0 = the carrier of X0
let A0 be non empty Subset of X; :: thesis: ( A0 is boundary & A0 is closed implies ex X0 being non empty strict closed boundary SubSpace of X st A0 = the carrier of X0 )
consider X0 being non empty strict SubSpace of X such that
A1: A0 = the carrier of X0 by TSEP_1:10;
assume ( A0 is boundary & A0 is closed ) ; :: thesis: ex X0 being non empty strict closed boundary SubSpace of X st A0 = the carrier of X0
then reconsider Y0 = X0 as non empty strict closed boundary SubSpace of X by A1, Th29, TSEP_1:11;
take Y0 ; :: thesis: A0 = the carrier of Y0
thus A0 = the carrier of Y0 by A1; :: thesis: verum