A valid basepair matching in the bonding graph of an RNA string $s$ is a noncrossing matching of basepair edges having the additional property that a basepair edge in the matching cannot connect symbols $s_j$ and $s_k$ unless $k \geq j+4$ (to prevent nearby nucleotides from base pairing).

See the figure below for an example of a valid matching of basepair edges in a bonding graph if we also allow wobble base pairs, followed by what the actual secondary structure of the folded strand of RNA will look like.