anchor.diff:    !---------------------------------------------------------------------------------------------------------------------------------
     endsubroutine add_tag
   
  +  ! private methods
     subroutine parse_from_string(self, source_string)
     !---------------------------------------------------------------------------------------------------------------------------------
     !< Parse xml data from string.
  
positive.code:     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.
      ! 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
  !-----------------------------------------------------------------------------------------------------------------------------------
  
negative.code:   !---------------------------------------------------------------------------------------------------------------------------------
    endfunction stringify
  
    elemental subroutine add_tag(self, tag)
    !---------------------------------------------------------------------------------------------------------------------------------
    !< Add tag to self%tag array.
  