anchor.diff:      procedure :: parse     !< Parse xml data from string or file.
       procedure :: tag_value !< Return tag value of tag named *tag_name*.
       procedure :: stringify !< Convert the whole file data into a string.
  +    procedure :: add_tag   !< Add tag to self%tag array.
       ! private methods
  -    procedure, private :: add_tag           !< Add tag to self%tag array.
       procedure, private :: parse_from_string !< Parse xml data from string.
   endtype xml_file
   !-----------------------------------------------------------------------------------------------------------------------------------
  
positive.code:   !---------------------------------------------------------------------------------------------------------------------------------
    endsubroutine add_tag
  
    subroutine parse_from_string(self, source_string)
    !---------------------------------------------------------------------------------------------------------------------------------
    !< Parse xml data from string.
  
negative.code:   !---------------------------------------------------------------------------------------------------------------------------------
    endfunction stringify
  
    elemental subroutine add_tag(self, tag)
    !---------------------------------------------------------------------------------------------------------------------------------
    !< Add tag to self%tag array.
  