Figure 3. Primitive pseudoknot tree constructed with 3 arcs.

Characters on the left of any internal node belong to the FIFO left end character queue, while those on its right belong to the right end character set. Nodes with pseudoknot pattern token ABCB cannot be extended because appending the right end character A violates rule 3, while appending character C violates rule 2. Similarly, for nodes with token ABCAC, appending character B also violates rule 3. At the lowest leaf node level, we obtain tokens ABCABC (pseudotrefoil) and ABACBC (kissing hairpin).