reserve X for non empty TopSpace,
  A,B for Subset of X;
reserve Y1,Y2 for non empty SubSpace of X;
reserve X1, X2 for non empty SubSpace of X;

theorem Th30:
  for A0 being non empty Subset of X st A0 is boundary ex X0 being
  strict SubSpace of X st X0 is boundary & A0 = the carrier of X0
proof
  let A0 be non empty Subset of X;
  assume
A1: A0 is boundary;
  consider X0 being strict non empty SubSpace of X such that
A2: A0 = the carrier of X0 by TSEP_1:10;
  take X0;
  thus thesis by A1,A2;
end;
