An RNA string s = s
= ACAGUGACU , and three corresponding folding s. (a) A folding of type I, obtained by adding the base-pair i • (j - 1) = 0 • 8 to a folding for si+1, j-1= s1,8. (b) A folding of type II, in which the last index 8 is paired to index 6. The folding is thus obtained by combining two independent foldings: one for the prefix s0,6, and one for the suffix s6,9. (c) A folding of type II, in which the last index 8 is unpaired. The folding is thus obtained by combining a folding for the prefix s0,8, and an empty folding for the suffix s8,9.