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