code: public static boolean isSingleReferenceType(Attribute attr) {
      AttributeType attrType = attr.getDataType();
      switch (attrType) {
        case CATEGORICAL:
        case FILE:
        case XREF:
          return true;
        case BOOL:
        case CATEGORICAL_MREF:
        case COMPOUND:
        case DATE:
        case DATE_TIME:
        case DECIMAL:
        case EMAIL:
        case ENUM:
        case HTML:
        case HYPERLINK:
        case INT:
        case LONG:
        case MREF:
        case ONE_TO_MANY:
        case SCRIPT:
        case STRING:
        case TEXT:
          return false;
        default:
          throw new UnexpectedEnumException(attrType);
      }
    }
docs: Returns whether the attribute type references single entities (e.g. is 'XREF').
  
  @param attr attribute
  @return true if an attribute references a single entity