Non-crystallographic symmetry (NCS) restraints are usually used
to keep identical or similar protomers close to each other
during the refinement. The restraints can be defined
in an file with the following format:
The first line contains the number of atoms within one subunit (Na) and
the number of identical subunits (Nsub). The following lines list
the AtomIDs within each subunit that are restraint. There must be exactly
Na * Nsub such entries.