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Get Boolean value indicating whether this instance of the Plural Field is the primary or
preferred value of for this field, e.g. the preferred mailing address or primary e-mail
address. Service Providers MUST NOT mark more than one instance of the same Plural Field as
primary="true", and MAY choose not to mark any fields as primary, if this information is not
available. For efficiency, Service Providers SHOULD NOT mark all non-primary fields with
primary="false", but should instead omit this sub-field for all non-primary instances.