Because of the no-(pseudo)knot condition 3 above, every base pair (i, j) subdivides a secondary structure into an interior and an exterior structure that do not interact with each other. This observation is the starting point of all dynamic programming approaches to RNA folding, see e.g. [32,33,37]. Including various classes of pseudoknots is feasible in dynamic programming approaches [38-40] at the expense of a dramatic increase in computational costs, which precludes the application of these approaches to large molecules such as most mRNAs.