Tetrahedral chirality with lone pairs unclear

[merkys]

SMILES+ specification uses @ and @@ to denote tetrahedral chirality. In some cases not all vertices of tetrahedron corresponds to atoms, but rather to lone pairs. Currently the specification does not specify whether it supports such chiralities or not.

I think SMILES+ could support that by saying that if one vertex of a tetrahedron is lone pair, then it is considered to be the first atom in the clockwise or anticlockwise accounting. If both lone pair and implicit hydrogen is present, then the order is first lone pair, then hydrogen. These suggestions are of course arbitrary and could be modified to the liking of the authors.

Originally reported on OpenSMILES tracker as https://github.com/timvdm/OpenSMILES/issues/9.