The basic Kabsch and Sander definitions for beta structure require that a residue must be involved in two hydrogen bonds (or bordered by residues involved in two hydrogen bonds) in order to be classified as in a `bridge'. Only residues in bridges can be built into ladders, and hence into sheets. However, residues frequently make a single hydrogen bond at the end of a ladder and would traditionally be considered as part of the sheet. The ladder_extension clause:
ladder_extension(L2,ladder(L1,CID1,CID2,Type)).
succeeds if the ladder may be extended to either the `left' or `right' end. L2 is the list L1 with the additional hydrogen bonded residue pair appended to the appropriate end or ends.
When Type = antiparallel, the extension is straightforward, eg: 20-119,21-118 might be extended to 19-120,20-119,21-118, or similarly at the right hand end. However, when Type = parallel, there is a problem, since the additional residue at the end of the ladder will be hydrogen bonded to a residue already in the ladder. As a consequence, the extended ladder list (L2) will contain a two references to a single residue on one strand. Eg. 20-90,21-91,22-92 is extended at the right hand end to: 20-90,21-91,22-92,22-93, or possibly: 20-90,21-91,22-92,23-92. Similarly at the left hand end.