The following example shows how to compute an SA-refined omit
map (Hodel, Kim, and Brünger, 1992). The omitted
region consists of an 8 Å sphere around residue 20. Other
selections can be specified here. The immediately
surrounding atoms, consisting of a 3 Å shell,
are restrained to avoid artificial movement into
the omitted region. The slow-cooling protocol is used for
simulated annealing (see Section 14.1.3).
