reserve a,b,c,d,e for Real;
reserve X,Y for set,
          Z for non empty set,
          r for Real,
          s for ExtReal,
          A for Subset of REAL,
          f for real-valued Function;

theorem Th13:
  a <= b implies {a,b} c= [.a,b.]
  proof
    assume
A1: a <= b;
    let x be object;
    assume x in {a,b};
    then x = a or x = b by TARSKI:def 2;
    hence thesis by A1,XXREAL_1:1;
  end;
