reserve FT for non empty RelStr;
reserve A for Subset of FT;
reserve T for non empty TopStruct;
reserve FMT for non empty FMT_Space_Str;
reserve x, y for Element of FMT;
reserve A, B, W, V for Subset of FMT;

theorem
  A` is Fo_open implies A is Fo_closed
proof
  assume A` is Fo_open;
  then
A1: (A`) = (A`)^Foi;
  (((A`)^Foi)`)` = (A^Fob)` by Th38;
  hence thesis by A1;
end;
