[Chimera-users] Selecting residues with zones when reference is absent

Hello, I would like to select all ligands that are within a distance of nucleic acids but above a distance from proteins. The selection command I have so far is: `select nucleic acid z<10 & protein z>10 & ligand` This works for PDBs that contain both nucleic acids and protein. However, some will only contain nucleic acids, and in this case the `protein z>10` condition will result in an empty selection even when there is a valid ligand. I was wondering if there was a way to check whether the model contains protein to run a different selection command for each case, or to use a different selection command which handles both cases. Thanks! Carlos G. Oliver