reserve T for non empty TopSpace,
  A, B for Subset of T,
  F, G for Subset-Family of T;
reserve x for Point of T;

theorem
  A^0 c= Der A
proof
  let x be object;
  assume
A1: x in A^0;
  then reconsider p = x as Point of T;
  p is_a_condensation_point_of A by A1,Def10;
  then p is_an_accumulation_point_of A by Th49;
  hence thesis by TOPGEN_1:16;
end;
