For more about topologically distinguishable points, see Topological distinguishability. In fact topological insulators are different from topologically ordered states defined in this article. This is usually achieved by topologically sorting expressions and updating values in topological order. Wormholes have been defined both "geometrically" and "topologically". Such pairs of polyhedra are still topologically or abstractly dual. Completely metrizable spaces are often called "topologically complete". Some DNA molecules are circular and are topologically constrained. The complexes constructed in structural DNA nanotechnology use topologically branched nucleic acid structures containing junctions. There are 257 topologically distinct "convex" octahedra, excluding mirror images. RP is called the real projective line, which is topologically equivalent to a circle. The equivalence class of topologically equivalent drawings on the sphere is called a planar map. There exist also two-dimensional polymers (2DP) which are composed of topologically planar repeat units. It is, however, R since the topologically distinguishable points are separable. The OGC standard definition requires a polygon to be topologically closed. The equivalence classes will be the classes of topologically indistinguishable points.