code: public Iterable<Attribute> getLookupAttributes() {
      Iterable<Attribute> lookupAttributes = getOwnLookupAttributes();
      EntityType extend = getExtends();
      if (extend != null) {
        lookupAttributes = concat(lookupAttributes, extend.getLookupAttributes());
      }
      return lookupAttributes;
    }
docs: Returns attributes that must be searched in case of xref/mref search
  
  @return lookup attributes