Fig. 4

A An RNA secondary structure. B Diagram representation of A. C A secondary structure with a rainbow arc (dash). D Loops in a secondary structure. The loop (gray) contains a distinguished maximal arc (1, 17). E A bistructure B is a collection of loops \(\{L_1, \ldots , L_9\}\). \(X = \{L_3, L_7, L_9\}\) (blue) is an irreducible substructure of B with its complement \({\overline{X}} = \{L_1, L_2, L_4, L_5, L_6, L_8\}\). We mark the exposed vertices \(E^X = V^X \cap V^{{\overline{X}}}\) in red. The closure of X is given by \({\tilde{X}} = \{L_2, L_3, L_4, L_6, L_7, L_8, L_9\}\)