code: def merge_attribute(attribute, newval, matches=nil)
        if attribute == :participant_medical_identifiers and newval then
          CaTissue::Participant.remove_phantom_medical_identifier(newval)
        end
        super
      end
docs: Overrides +CaRuby::Mergable.merge_attribute+ to work around the caTissue
    bugs described in {CaTissue::Participant.remove_phantom_medical_identifier}.