reserve FT for non empty RelStr;
reserve x, y, z for Element of FT;

theorem Th3:
  FT{0} is filled
proof
  let x be Element of FT{0};
  x = 0 & [0,0] in SinglRel 0 by TARSKI:def 1;
  hence thesis by RELAT_1:def 13;
end;
