Syntax:

ring ( neta )

Denotes that the target atom (if specified in the root of the description) or a bound atom (if used inside the associated bracketed part) should be present in a ring structure of some kind. The 'ring' keyword alone simply demands that the atom is present inside a ring of some size, but may take an optional bracketed part describing more fully the number of atoms in the ring, and the individual nature of each of these atoms. Within the bracketed part, bound atoms may be specified as usual in the contained neta, but the connection type is irrelevant as it is only the presence of those particular atoms within the ring that is considered important.

Bound atom descriptions given inside the bracketed part should again be listed in order of decreasing complexity. Multiple rings may be specified with separate 'ring' keywords, allowing the location of fused ring atoms.

For example: