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 closed implies A` is open
proof
  assume A is closed;
  then
A1: A = A^b;
  A^b = ((A`)^i)` by Th16;
  hence thesis by A1;
end;
