code: def link(self, ref: Optional[Union[str, Element]], *, after_link: str = None, use_desc: bool=False,
               add_subset: bool=True) -> str:
          
          obj = self.obj_for(ref) if isinstance(ref, str) else ref
          nl = '\n'
          if isinstance(obj, str) or obj is None or not self.is_secondary_ref(obj.name):
              return self.bbin(ref)
          if isinstance(obj, SlotDefinition):
              link_name = ((be(obj.domain) + '.') if obj.alias else '') + self.aliased_slot_name(obj)
              link_ref = underscore(obj.name)
          else:
              link_name = self.obj_name(obj)
              link_ref = link_name
          desc = self.desc_for(obj, use_desc)
          return f'[{link_name}]' \
                 f'({link_ref}.{self.format})' + \
                   (f' *subsets*: ({"| ".join(obj.in_subset)})' if add_subset and obj.in_subset else '') + \
                   (f' {after_link} ' if after_link else '') + (f' - {desc.split(nl)[0]}' if desc else '')
docs: Create a link to ref if appropriate.
  
          @param ref: the name or value of a class, slot, type or the name of a built in type.
          @param after_link: Text to put between link and description
          @param use_desc: True means append a description after the link if available
          @param add_subset: True means add any subset information that is available
          @return: