
theorem
  for L being non empty RelStr, X being Subset of L holds X "\/" {}L = {}
proof
  let L be non empty RelStr, X be Subset of L;
  thus X "\/" {}L c= {}
  proof
    let a be object;
    assume a in X "\/" {}L;
    then ex s,t be Element of L st a = s "\/" t & s in X & t in {}L;
    hence thesis;
  end;
  thus thesis;
end;
