Fig. 1.

Stochastic backtrack procedure for a given substructure S. Either the first position is left unpaired (top), a base pair is formed between the two extremities, stacking onto an exterior base pair (middle) or paired without creating a stacking, defining two regions on which subsequent recursive calls are needed (bottom). For the empty structure (omitted here), the empty sequence is returned. Positions indicated in red are assigned at the current stage of the backtrack