  We refine the Polyak-Viro Gauss diagram formula for the Vassiliev invariant
of order two in a very simple way for the 2-cable of a framed long knot.
Surprisingly, the resulting isotopy invariant of framed knots can detect
already the non-invertibility of knots. This makes the natural generalization
of our invariant for all n-component string link satellites of a framed long
knot to a candidate for distinguishing all classical knots.
