reserve A for RelStr;
reserve X for non empty set;
reserve PX,PY,PZ,Y,a,b,c,x,y for set;
reserve S1,S2 for Subset of Y;

theorem
  {{}} is hierarchic
proof
  let x,y be set such that
A1: x in {{}} and
 y in {{}};
  x = {} by A1,TARSKI:def 1;
  hence thesis;
end;
