reserve FT for non empty RelStr,
  A,B,C for Subset of FT;

theorem Th1:
  ({}FT)^b={}
proof
  assume not thesis;
  then consider x being object such that
A1: x in ({}FT)^b by XBOOLE_0:def 1;
  ex y being Element of FT st x=y & U_FT y meets {}FT by A1;
  hence contradiction;
end;
