reserve FT for non empty RelStr;
reserve x, y, z for Element of FT;
reserve A for Subset of FT;
reserve F for Subset of FT;

theorem
  A is open implies A` is closed
proof
  assume A is open;
  then
A1: A = A^i;
  A^i = ((A`)^b)` by Th17;
  hence thesis by A1;
end;
