Some PDB geometries have two or more possible positions for some atoms
(positional disorder). These are indicated by the letters 'A', 'B', etc.,
before the three letter abbreviation for the residue. Only one option for a set
atoms can be selected. To select all atoms of type 'A,' use ALT_A=A; to
select all those of type 'B,' use ALT_A=B, etc.
See also: PDB, CHAINS