theorem
  x <==> y implies x <<01>> y
  proof
    assume
A1: x <==> y;
    per cases by A1;
    suppose
A2:   x ==> y;
      take x;
      thus x <=01= x & x =01=> y by A2;
    end;
    suppose
A3:   x <== y;
      take y;
      thus x <=01= y & y =01=> y by A3;
    end;
  end;
