anchor.diff:  		      DateTime
   	     end,
   
  -    Dtend_Duration = 
  +    Dtend_Duration =
   	case {get_attribute(Cond, dtend), get_attribute(Cond, duration)} of
   	    {'#no_value', '#no_value' } ->
   		throw({error, time_tag_no_duration_or_dtend_attribute_supplied});
  
positive.code:     lists:sort(ValidInts).
  
  %%--------------------------------------------------------------------
  %% Function: 
  %% Descrip.: 
  %% Returns :           
  %%--------------------------------------------------------------------
  get_clear_value("yes") ->
      yes;
  
negative.code: %%       contained / pointed to.
  %%
  %% To do:
  %% * add more descriptive error messages, include line number,
  %%   tag/attribute name and value encountered
  %% * the current implementation doesn't reject all unexpected sub tags
  %%   and attributes - they are simply ignored. A error should be
  %%   generated when they are encountered so that misspelt and
  %%   unsupported tags/attributes can be detected - this is mainly a
  %%   problem if scripts are hand coded or if the use extensions to
  %%   RFC 3880
  %% - code checks if sub tags name is allowed as element of a tag
  %% - check for otherwise as last element (if otherwise sub tag is
  %%   used) is done
  %% - checks are done for sub tags that occur multiple time when they
  %%   should only occur 0-1 times
  %% * add time zone and summer/winter time support
  %% * there is no check for overlapping time intervals - the cost (in
  %%   time) of this feature is high compared to gain received from
  %%   implementing it
  %% * all atoms could be changed into macros so that the compiler
  %%   catches misspelt names - "undefined macro used"
  %%--------------------------------------------------------------------
  
  