              The value of _phasing_MIR_der_site.id must uniquely identify each\
               site in each derivative in the PHASING_MIR_DER_SITE list.\
\
               The atom identifiers need not be unique over all sites in all\
               derivatives; they need only be unique for each site in each\
               derivative.\
\
               Note that this item need not be a number; it can be any unique\
               identifier.