Given two structures in bracket notation, a more stringent secondary structure evaluation can be carried out by considering all pairs of positions, and evaluate the agreement in their structural notation (i.e., dots, opening and closing brackets) in both structures. For each pair of positions (i, j), there are five possible cases. The two positions can be unpaired (4) or paired with each other (1) . Furthermore, only the left (2) (resp. right (5)) position can have an opening (resp. closing) bracket. Finally, both positions can be paired, but with different partnersi (3).