reserve T for non empty RelStr,
  A,B for Subset of T,
  x,x2,y,z for Element of T;

theorem Th6:
  A c= B implies A^d c= B^d
proof
  assume A c= B;
  then
A1: B` c= A` by SUBSET_1:12;
  let z be object;
  assume
A2: z in A^d;
  then for y st y in B` holds not z in U_FT y by A1,Th2;
  hence thesis by A2;
end;
