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

theorem Th2:
  x in A^d iff for y st y in A` holds not x in U_FT y
proof
  thus x in A^d implies for y st y in A` holds not x in U_FT y
  proof
    assume x in A^d;
    then ex y st y = x & for z st z in A` holds not y in U_FT z;
    hence thesis;
  end;
  assume for z st z in A` holds not x in U_FT z;
  hence thesis;
end;
