Of the twist knots, only the unknot and the stevedore knot are slice knots. Mathematically, this means that a trefoil knot is not isotopic to the unknot. Intuitively, the unknot is a closed loop of rope without a knot tied into it. The Shakespeare knot, a type of decorative unknot, is a heraldic knot. Similarly, the unknot is the identity element with respect to the knot sum operation. In particular, since the unknot is not tricolorable, any tricolorable knot is necessarily nontrivial. In the ordinary sense of the word, the unknot is not "knotted" at all. The conjecture states that all knots, except the unknot, have Property P. In three dimensions, the unknot cannot be written as the sum of two non-trivial knots. Both counts found 1701936 prime knots (including the unknot) with up to 16 crossings. In 2010, Kronheimer and Mrowka proved that the Khovanov homology detects the unknot. The simple toroidal vortex, represented by the circular "unknot" 0, was thought to represent hydrogen. Human carbonic anhydrase was identified to have a low unknot score (22). The simplest such invariant is tricolorability&colon; the trefoil is tricolorable, but the unknot is not. Unknot recognition is known to be in both NP and co-NP.