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
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
Overrides +CaRuby::Mergable.merge_attribute+ to work around the caTissue
  bugs described in {CaTissue::Participant.remove_phantom_medical_identifier}.