Some PDB geometries have two or more possible residues at a given site
(structural disorder). These are indicated by the letters 'A', 'B', etc.,
after the three letter abbreviation for the residue. Only one set of residues can be
selected. To select all residues of type 'A,' use ALT_R=A; to select
all those of type 'B,' use ALT_R=B, etc. Sometimes a space is used
instead of 'A', in which case the keyword would be "ALT_R= " (note the
space after the equals sign).